Tech Tree

A staple of almost every strategy game is the ability for you to unlock new abilities for your units, or new unit classes entirely, by spending time and resources on scientific research instead of just bashing your opponent into submission with your existing ones.

Exactly how the tech tree works varies greatly depending on the game genre.

In Real Time Strategy games, research is usually represented by specialized units or structures, with the pace of new tech development decided by how many of these the player has on the field. Research units often have weak attacking abilities (If they can attack at all) and must be protected from harm. Smart players and AIs will, of course, constantly be after these units.

New technologies typically allow for better armor to take more damage before dying, faster ground speeds, weapons that do more damage per hit, and increased sight and accuracy bonuses.

Tech upgrades are usually dressed to look genre-appropriate for the game. Researching advanced radar tech for your helicopters in one game will be instantly recognizable as granting your Paladins "Holy Sight" in another.

Another key feature is that they usually follow a set order in which they must be researched. To give your troops armor-piercing shell upgrades, you may have to first research the advanced artillery tech, which in turn can't be done until you've finished researching basic cannon tech, etc...

Some games allow players greater influence over their gaming economies, and they can pour extra money into certain research projects to get them done faster while completing less urgent ones at their leisure.

Other times the development speed is static, and all that's required is that the player have enough gold, tiberium, wood, mana or whatever is needed to pay the upgrade costs for that tech.

The use of tech trees in 4X games is quite different. Tech trees typically do not have an on-map representation. They are a function of the empire itself. In such games, each empire's cities (or equivalent) provides some portion of research that is pooled until the civilization researches a particular technology. The rewards for a tech are improvements for "cities", new units or unit equipments, bonuses for a civilization researching them, or other such things.

Some Role-Playing Games have the similar "Feat Tree", where at character creation time and at every Nth Level Up the player gets to choose new traits and abilities (feats) for their character, with some feats requiring other feats to unlock.

Tech trees are one of the big points where historians pick at games. It's a hierarchical view of science, research, and history. When compared with actual history, tech trees are wrong. On the other hand, the few attempts at doing something different have wound up pulled from any final version, with good reason.

Either way, researched technologies in many games, most often Real Time Strategy, have an annoying habit of disappearing once the current level/mission is completed, forcing you to spend time researching them all again in the very next round.

See also You Have Researched Breathing, Reinventing the Wheel. A Tech Tree may be prone to Interface Spoilers, or conversely, it may become a Guide Dang It to make an informed decision.

4X

 * The 1995 classic Ascendancy from The Logic Factory has a three-dimensional Tech Tree that is simply stunning in its scope and variety. Unlike many games, it is not necessary (or even possible) for a player to acquire all technologies to win—the tree branches in many wildly different and interesting directions, allowing races to specialize in something strange and unusual yet still have a strategically versatile "power set".
 * Slightly ahead of Aaron Hall's Space Empires series then, although this is probably more a case of parallel development than actual copying since the slightly less advanced Space Empires II was released in the same year, but was a big advancement on the first game.
 * Galactic Civilizations II has a large but fairly simple tech tree; all technologies have a single prerequisite. A long, time-intensive branch dealing with philosophy and the nature of existence has no mechanical in-game benefits, but researching the final step of this branch triggers the research victory condition: the entire population of the player's race transcends into Energy Beings.
 * The branch in question consists of five increasingly expensive (in terms of research points) parts, to wit: "Deeper Knowledge," "Galactic Understanding," "Near Omniscience," "Beyond Mortality," and, finally, "Technology Victory." The Torians have a similar branch that does reward +5% research for each step along the way, to make up for their limited tech buildings.
 * As of the Twilight of the Arnor expansion, there's a whole orchard of tech trees, for each of the different major races, emphasizing their different strengths and weaknesses and going deeper into the history and character of each race.
 * Civilization, as the name implies, has an absurd Tech Tree that spans all of human history from the stone age to the space age and beyond. One of the final Wonders of Civilization grants instant victory upon completion of mankind's first interstellar spaceship, which leads to:
 * Sid Meier's Alpha Centauri also has a really complex tech tree, like most Firaxis games, where the player must research all sorts of future technology until he can reach the Threshold of Transcendence. Once the player finishes the Ascent to Transcendence project, the player's civilization will Ascend to a Higher Plane of Existence.
 * Definitely in Civilization 2 (and probably in the original Civilization) the cost of getting new technologies depends on the number of technologies you already have - so getting even one technology that you don't need is extremely expensive. As a result good players had absurd technology trees. Freeciv fixes this problem by making the cost of technology depend only on its position in the tree.
 * Civilization Revolutions has a much shorter tech tree (48 techs). It is occasionally possible to research a higher technology without all of the prerequisites.
 * But Civilization 4, the game that Revolutions is most closely related to, has a larger tech tree; Revolutions is in some ways a stripped-down version of Civ 4.
 * In Civilization V, in order to research any technology that has multiple prerequisites, all prerequisite techs - not one, not some, but all - must be researched first. On the one hand, it makes for somewhat more realistic tech progression. On the other hand, this makes it impossible to skip over unneeded technologies.
 * In the short-term one can "beeline" one branch of technology without touching other parts, though a balanced civ will find most techs tempting in one way or another. This results in some odd combinations, like being able to make a chariot archer without archery, or the giant death robot without robotics.
 * The base Space Empires games already have sizable tech trees that grow larger with every iteration of the game. They are also quite moddable, and modders have created veritable jungles of tech trees s a result.
 * Sword of the Stars has a large interconnected Tech Tree. With the added factor of each race getting various percentage chances to have the various technologies in their tree, meaning that no two games, or players, are completely identical tech wise.
 * The Master of Orion series uses slightly different tech trees in each game:
 * In the original game, players invest research points to gain generic tech levels, which unlock various weapons, colony improvements, etc.
 * In the second game, there are several trees to research (Power sources, Physics, Chemistry, etc.) and each tree has usually has multiple tech choices at each level. Most races can only receive one tech per level through research, and races with the "Uncreative" trait cannot choose which one they get. Creative races get everything for the same price a regular race pays for one item.
 * The third game is probably different somehow, but no one is willing to admit to playing it.

Hack and Slash

 * This can also be seen in Diablo 2. Sometimes this makes sense, like how a Sorceress has to learn the basic Ice Bolt spell before learning the more advanced Blizzard. Other times, like how the Barbarian has to learn Leap (what it sounds like) before Whirlwind (a spinning blade attack), it's obvious they're just prerequisites for prerequisites' sake.

MMORPGs

 * Each World of Warcraft class has its own, specialized tech tree, with abilities that can only be gained by putting points into it. One point per level gained from level 10 onwards.
 * Now one for every other level, to cut down and the number of compulsory but boring and "increases your X damage by Y%" talents.
 * This trope is a huge point of the MMORPG A Tale In The Desert, where practically the entire playerbase rushes to unlock technologies as soon as they're available, no matter how useful they turn out to be in the end.
 * EVE Online has tech trees as well. In order to fly certain ships or use specific modules, you need to research a multitude of skills, some of which take weeks to learn.
 * World of Tanks uses the Tech Tree as a form of Experience level AND tech tree. As you fight battles you earn XP and credits, which you use to upgrade your existing vehicle, and then purchase the next higher tier vehicle or vehicles in line.

Real Time Strategy

 * Age of Empires III has an interesting take on this trope. While it features a relatively mundane tech tree for most of its buildings and units, players can customise available shipments from their Home City by setting up a virtual "deck". This means that two players' play styles can vary dramatically even if they use the same nation, depending on what types of shipments they choose to make available.
 * Hegemonia: Legions of Iron had a research system that needed to be managed along with the players units. Since the multiplayer mode made it so that players on the same team were commanders of the same fleet, players could split the duties by having one team member manage colonies and research while others managed combat.
 * Warzone 2100 is absolutely crazy about this, since it has several hundred techs, probably range up to thousands, to be researched. Good thing it averts Reinventing the Wheel.
 * Paradox Interactive has tech trees that tends to be rather complex.
 * Europa Universalis has tech "levels" based on four different categories (Land, Naval, production and trade techs) and a fifth category, "stability" that isn't tech per se but gets money from the same budget. EU3 adds a fifth tech category ("government") tech is researched simply by spending money from your budget, but the tech cost is modified by... Well, a shitload of different factors. Different levels unlock different types of upgrades and buildings.
 * Hearts of Iron 2, set during World War Two, has a more conventional tech tree, with advances leading to further advances in various categories. All of them have a "historical year" which make them much more expensive to research before their time. Research is carried out by country-specific "Tech Teams" such as Boeing or Wernher von Braun. Of particular note is the "Secret" category, which consists of tiny groups of unrelated advances - electronic computers, nuclear fission, rocket interceptors - that become available a short while after their prerequisites from other categories are reached.
 * By contrast, Hearts of Iron (the previous game in that type) has a vastly more complicated tech tree with at least several hundred techs. Moreover, you research the technology using the same industrial capacity you use to build units, which creates conflicts of interest.
 * Victoria: An Empire Under The Sun has a relatively simple tech tree with different "levels" of technology, but each technology "level" will, at certain randomized intervals, trigger "inventions" which provide most of the benefits. Research is based primary on your country's literacy rating but also on the makeup of your society (clergy and clerks providing the most research output, although the "darwinism" invention crashes clergy's research output and doubles that of clerks)
 * Crusader Kings is set in medieval times and uses a "directed fluke" system: you pick techs and discover them in your capital a random number of years later. However, all technologies are features of particular territories, not their owners. Paris will not stop having an university because it's inherited by a monarch whose home doesn't, nor will Finland revolutionized its agriculture just by falling into the hands of a country that has. Techs spread over borders and trade routes, affected by politics and infrastructure.
 * StarCraft requires specific buildings to produce units and research upgrades, and most buildings have other buildings as prerequisites for construction. The structure of the building hierarchy varies for each of the three races, with zerg having a tier based system, protoss having 3 largely independent tech paths, and Terran having a shorter central path with several large 'branches' for better units in certain classes. Upgrades offer either a unit-specific ability or improvement (such as increased attack range for the zerg hydralisk or a short range teleport for protoss stalkers) or a generic attack/defense boost for a class of units (e.g. increased attack power for all air units). Upgrades cost a fixed amount of resources and take a fixed amount of time to research once purchased.
 * One of the daddies of them all was Utopia, back in 1991, although it may have been pre-empted by Reach for the Stars, released eight years earlier. Utopia was followed up by K240 in which the 'research' is already done, it just has to be purchased (but making money requires selling your precious ores which you may want to use to build starships).
 * Red Alert 3 is odd about this, all sides have to build a tech building to get the most advances stuff but Allies and Imperial have to pay for further upgrades, either in the form of security clearance for the construction yard (allies) or upgrades for individual buildings (imperial), the soviets just need a tech building.
 * The Total War series used a tech-tree without actual research. Better versions of existing buildings become available when a city grows to a certain size, and in turn allow recruiting better units. Therefore, reaching the top tier of production usually involved protecting your kingdom long enough to let its cities grow big.
 * In Empire: Total War, a more conventional Tech Tree was introduced - you could study new ideas in three separate trees (social, economic, military), which in turn unlocked new buildings, units, strategic advantages, and even new actions to perform during battles. Unfortunately, the designers went too heavy on the Acceptable Breaks From Reality, allowing players to research things like ocean-going steam ships and rifling in the early 18th century. It didn't help that many players (and not just the veteran fans of the series) found the game to be sub-par in several other respects, so Internet Backdraft ensued.
 * The expansion/sequel Napoleon: Total War used a tech-tree as well, and in a case of subtle Retcon featured a Tech Tree very similar to that of its predecessor (that is, containing many of the same techs), except now it was the 19th century and thus more suitable... except many of the technologies on the tree were still from way beyond of Napoleon's time and the game's scope.
 * Achron has an interesting spin on this. Each faction has a very small, very flat tech tree (usually amounting to around six upgrades or so) and they are explained away as "improving infrastructure" (as opposed to "research"). The interesting bit is the way this interacts with the time travel mechanics: It's possible to arm your units with weaponry from the future, or reinforce your army with units you can't technically build yet.
 * Eight Realms has researchable tech trees available via the Library or University.
 * Jeff Wayne's War of the Worlds has a mostly hierarchical tech tree, though some things are instead unlocked as counters, AA Guns after encountering flying machines for example.

Role-Playing Game

 * SaGa Frontier has an absolutely immense Tech Tree, with over 100 abilities that can be learned from each other with various degrees of success depending on what ability is being used. And that's not even including the various combination attacks that can be performed between party members.
 * All of Arcanum's technology trees were single-pathed, non-cumulative skills that allowed you to craft something. All crafting trees had a single theme, some useful only at the intended level of reachability, some upgradeable at higher tiers, some upgradeable with cross-tree synergy, and some worthless for anything except to unlock higher tiers. But there were some trees that the top skill is the only reason to invest character points, and others where the top skill is found to be practically worthless; that is worthless under any circumstance that you could've imagined it being valuable for.
 * Legend of Mana starts you off with about eight basic moves (and no super moves); learning new moves involves you putting moves together, getting experience, and hoping the Random Number God is merciful.

Simulation Game

 * Syndicate required you to research new weapons and upgrades before you could buy them for your team of cyborgs between missions. Which is fair enough (laser rifles and advanced bionic arms probably take a bit of working out), but it got a bit silly when your evil 21st century corporation could field a squad of bionic hitmen but you couldn't even build uzis for them until you'd done the research.
 * A Kingdom For Keflings has a blueprint tree - completing a building unlocks those below it. The tree occasionally branches, and it's often impossible to unlock multiple branches in one kingdom.
 * Pretty-much the entire point of the open-source game Endgame: Singularity is to research the entire Tech Tree while not being discovered. Sounds easy, right? Well, not in the early stages of the game. Once you have Quantum Computers, however...
 * The Naval Ops series uses a set of Tech Trees to organize research. One tree for torpedoes, one for lasers, and so one. Figuring out just how to unlock certain pieces of equipment can be an exercise in Guide Dang It.

Tower Defense

 * Most Tower Defense games require that the player first build lesser towers before gaining access to the higher-level ones. A few of the more clever ones instead have the player escalate through the Tech Tree by "fusing" their lower towers together into exponentially stronger towers that usually have some characteristics of the previous ones.

Turn-Based Strategy

 * The old turn-based strategy game Deadlock and its sequel have an elaborate tech tree that works just like that. It's also possible to buy technologies on the black market (how the dealers got hold of super-advanced technology is never really explained).
 * It's because you and the black market aliens all had the tech beforehand: the races that contested the planet all agreed that each colony had to start off at with limited resources and technology. You're not inventing the technology; you're developing the tools to make the technology.
 * Heroes of Might and Magic has blueprint trees for each town type. In later games each town has a different path when it comes to horde production. To make towns more balanced, some of the towns with overall weaker units can build their higher level horde production buildings much earlier than towns with more powerful armies (ie the Stronghold in III being able to build the Behemoth structure in three days from scratch). IV forces the player to choose which 2nd, 3rd, or 4th level units they want from a town. V complicated the formula from III with town levels: a town has to have a certain number of buildings already built before it can build higher level buildings like a city hall or capitol. Hero development in IV and V has a tech tree of sorts with certain skills needed to unlock new ones (and a very specific path is needed to unlock a Hero's Ultimate Skill in V).
 * Each race in Disciples 2: Dark Prophecy has a different blueprint tree for each type of unit (ranged, melee, magic, and support). Adding another layer of strategy, once you choose which path to take for certain unit types for certain races you can't build the other path. For example, the Empire's initial support unit, the Acolyte, can be upgraded into either a more powerful single healer unit or a weaker mass-healer, but once you pick one upgrade structure to build you can't build the other one.

Tabletop Games

 * GURPS has a sort of "magic" tree. In order to learn advanced spells you have to learn simpler spells that presumably give the character the knowledge needed to understand the more complicated ones.
 * Magic: The Gathering has the "Level up" mechanic from the Rise of the Eldrazi expansion, which allows you to sink additional mana into the creatures you already have to upgrade them to stronger versions.
 * Starfire introduced Tech Trees starting in 4th Edition, replacing their old system of Technology Levels.
 * Dungeons & Dragons 3.X had the Feat Tree example, and actually used those words to describe it. Feats were granted at level 1, level 3, and every 3 levels thereafter. Many Feats required other Feats, and there were even some deliberately weak Feats (for example, Endurance gave a +4 bonus to saves to resist exhaustion and the ability to sleep in Medium armor, which would be relevant only if the GM was a whole-hearted Rules Lawyer) that were primarily used as prerequisites for above-average Feats to keep them balanced (Endurance was a prerequisite for such Feats as Diehard (lower penalties for being below 0 hit points and takes more damage to kill you) and Steadfast Determination (use the CON score you were prioritizing for both Fortitude and Will Saves, making the WIS score you were using as a Dump Stat directly govern nothing)). Being able to climb a Feat Tree was pretty much all the Fighter class had going for it, as its only class feature was a bonus Feat (taken from a very long list) at first level and every even level. As such, there were several Feat Trees created exclusively for Fighters, even requiring a minimum Fighter class level to take.