Level Scaling

A specific form of Anti-Grinding, usually seen in RPGs and Roguelikes, though it could potentially be used in any game with Character Levels, Level Scaling is where the world (or specific areas) levels up with you, to provide a constant challenge, primarily by upping your foes' stats.

When done well, it does exactly as intended, providing a constant challenge that keeps the game fun, and will keep itself largely unobtrusive. When done badly, it can head straight into Empty Levels, and/or cause such fun things as bandits with crazy powerful weapons and armor that they could retire in luxury just by selling, trying to kill you. Underleveling (purposefully keeping yourself or your party at a low level) can become a viable tactic (and, under certain circumstances, a Game Breaker) if this trope is in effect.

This is becoming more common in RPGs, especially sandbox-style RPGs, as it makes it easier to keep the player challenged, while limiting the need for predicting the level the player will be at when they reach a certain point.

Contrast Sorting Algorithm of Evil, where the enemies get tougher as you go along, regardless of your own level.

Sub-Trope of Dynamic Difficulty.

FPS

 * Borderlands is similar to the Dragon Age example below in that each area has minimum and maximum level, and the enemies you encounter are at your level, or their minimum or maximum. Of course, no one's quite sure how the level scaling for co-op is done.
 * Crawmerax is also set up to be five levels higher than you, when you meet him. Good luck...

MMORPG

 * In the game Kingdom of Loathing, there are several times where a monster will scale to your level, that is, their stats are equal to yours, plus a certain amount, including the optional boss, a past incarnation of the final boss, the Naughty Sorceress (later reduced to a static difficulty), and all holiday-related monsters.
 * In Dark Age of Camelot, an instanced dungeon's difficulty is scaled by its level range, your level, and the number of players in the group. If you enter an instance that's level appropriate for you, the mobs will be relatively easy to kill and complete solo. As you add more members however, the enemy NPC's levels will also increase to increase its difficulty as well as rewards.
 * Played mostly straight in City of Heroes. While enemies in open world areas have fixed levels, most missions are instanced, and the instances are scaled to player levels and group sizes. In case of the flashback system that allows high-level heroes to revisit low-level missions, the player is scaled in level to match the mission difficulty.

Roguelikes

 * Nethack determines enemy level by averaging your level with your current dungeon depth.
 * Beneath Apple Manor, which actually predated Rogue by two years. Each time you entered a new level the creatures' hit points and damage done were increased to be proportionate to your damage done and hit points, but you could spend Experience Points to increase your stats at any time. This meant that you started off a level fairly vulnerable to monster attacks but became more powerful over the course of the level, easily defeating monsters at the end.
 * ADOM scales a species' level by the number of that species of monster that's been killed. This means that Enemy Summoners that create endless swarms of a single weak species (like werejackals which summon hordes of jackals) will lead to that species soon becoming very tough.
 * Also, the 'Small Cave' starter dungeon's enemies scale by your level, but in a way that will cause them to massively outpace you if you don't get through it quickly.

RPG

 * Final Fantasy examples:
 * Final Fantasy Tactics Advance and A2 base random encounters on your clan's average level, which can be exploited, by having a bunch of low level people in your clan, with a few high level people that you generally use.
 * Final Fantasy VIII was the Trope Codifier for RPGs. It matched a monster's level to your party's level, with the monsters automatically learning new (more dangerous) techniques. Savvy players figured out that using the "card" ability allowed the player to defeat enemies without collecting the base Experience Points, thus enabling their characters to grow more powerful (from the other victory spoils) while enemies remained the same. However, a number of rare items are impossible to acquire in this way.
 * Actually, the 'LV UP' and "LV DOWN' abilities allow you to forcefully change a monster's level to whatever range gives them the item you want. Combined with the item refinement system no items are actually unobtainable, except ones that are exclusive to the Pocketstation minigame.
 * In Final Fantasy Crystal Chronicles, when collecting second and third drops of myrrh from a given area, the enemies have grown stronger since the player's previous visit, new enemies appear, and bosses unveil more powerful attacks.
 * Final Fantasy Tactics bases non-story battles on your party's levels, which can be a problem, because while monsters gain almost all of their stats from leveling up, humans, especially melee fighters, gain most of their stats from equipment.
 * Dragon Age has level scaling, set up where each enemy has a minimum and maximum level, and being limited to particular areas, though some will always level with you.
 * Also your allies will always be within one level of your character, so they never fall too far behind.
 * The bosses in Lunar: Silver Star Story and it's PSP remake Silver Star Harmony have stats that scale with the hero's level, so it's easier to defeat some bosses at low levels. Released in 1992, this would make Lunar the Ur Example of RPG level-scaling.
 * What also learned from Lunar was the Tactics Ogre sub-series. In that game, many of the enemies in the random encounters scaled with your party's level. However; story battles would eventually cap at a certain point so one could level to 50 and just throw rocks at the level 20 enemies and kill them. It was also possible to exploit this in Knight of Lodis, where it was actually scaled off of Alphonse's level, so keep him three levels below the rest of the party and they carry him to victory.
 * The PSP version of Tactics Ogre also uses this trope; but they discourage you from simply grinding your characters to level 50 because the levels are based off of your class's levels, instead of characters gaining EXP individually. So you can hit level 50 in the first chapter theoretically, but unless you somehow managed to get the other classes that early, they'd be at level one so if you ever tried to train them during a random encounter, you'd be constantly reviving them because the AI will immediately target them.
 * The 7th Saga does this when fighting other playable characters as bosses. Those enemies are matched to be at exactly your player character's level, making Level Grinding largely pointless. If you lose against them, they steal whatever runes you were holding on you when you fought them, and can use them against you for the inevitable rematch.
 * Worse than pointless, in fact. In the North American release, the stat points gained when your character levelled up were reduced, but the enemy stats were unchanged, resulting in incredibly hard fights if you were too "powerful".
 * "Incredibly hard" is something of an understatement. These battles often reached into flat-out Unwinnable territory, even when you cheated.
 * In The Last Remnant the enemies do scale to your level(even though you don't technically have one), but have a certain cap when they stop levelling, Special encounters however do not scale and have set statistics.
 * In Baldurs Gate 2 the types of random enemies adjusted to match your level. In extreme cases this could lead to fighting through a dungeon of random superpowerful liches to fight the comparatively pathetic ostensible boss.
 * In Anachronox, enemies would, at certain points, be scaled to match Boots' (the main character) current level. Thus, enemies would get easier to fight in a certain dungeon, then get tough again upon leaving. As Boots is in your party non-stop, this made level-grinding something to avoid, as Boots would surpass his fellows and render them useless against even mooks. The game made an effort to compensate by similarly increasing the levels of characters that fell behind, but didn't do so enough.
 * In Marvel Avengers Alliance, regular missions play at a set level, but the enemies in Bonus/Special missions are set at the player character's level, making them difficult (but rewarding) for everyone.

Wide Open Sandbox

 * Morrowind has this to a limited extent. While foes you'll encounter in caves will always have a fixed level, some creatures on the road will be matched to your level, instead. This, of course, has the advantage of making travel a constant danger.
 * Also, the number of enemies you encounter increases, which means that you'll be attacked almost constantly at higher levels. Worse, you'll be attacked largely by enemies that are not strong enough to provide a challenge, like Cliff Racers. This might explain Bethesda's decision to take a different approach with Oblivion
 * Oblivion is an excellent example of how not to do level scaling. Characters that are not combat-oriented will find that enemies will still scale to their level, as if to emphasize their inability to fight. Characters that are combat-oriented will find that, eventually, most fights become a mindless slugfest to try to deplete the opponent's massive HP, with no real challenge or risk of death. Enemy equipment also scales up, totally unbalancing the game's economy. And the worst part? Named NPCs do not scale with you, so if you fight through the Siege of Kvatch at level 4 your companions will probably do fine, but if you try it at level 20, They'll get slaughtered. Underleveling often becomes the only way to survive. It's an infamous enough example of level scaling done badly that many people refer to badly implemented level scaling as "Oblivion Syndrome."
 * Fallout 3 proves that Bethesda learned their lesson from Oblivion's screw up. The level scaling is based on your level when you enter an area and is never adjusted again for that area, so in the starting areas, you'll deal with easy enemies, and as you get stronger and go further out, the enemies will also get stronger, but if you back to the beginning areas, you'll be dealing with the weak enemies again.
 * In addition, enemies are prebuilt to a certain level and pulled off a list to set what's appropriate. By contrast, Oblivion uses the same basic enemy at levels 8 and 20, but improves his stats and equipment. Fallout spawns a level 8 enemy when you enter an area at level 8, and a different level 20 enemy when you enter it at level 20. So you'll be fighting Enclave Troopers in Tesla armor at level 20, instead of Raiders who happen to be wielding Gatling Lasers while ensconced within Powered Armor.
 * Raiders get upgrades as well. At level 5 they carry mainly pistols and hunting rifles. At level 20, they're equipped with missile launchers and assault rifles instead.
 * Skyrim has level scaling "like Fallout 3's, not Oblivion's," And the Fandom Rejoiced. Most enemies simply get replaced by tougher variants in high level areas, while some do directly scale with player level. Random loot also scales. Level-scaling is still not perfect, as it is quite easy to grind non-essential skills and end up facing high level opponents at every newly found area.
 * Critical Miss demonstrates the imperfections.
 * Dead Island has scaling similiar to Oblivion. Whatever level you are, the enemies will be. Your health increases, but their damages increases to do roughly the same amount percentage wise. Their health increases, but you can equip stronger weapons to do the same back to them.

Other Genres

 * In Muramasa the Demon Blade, enemies are always scaled to correlate to your level. Even when overleveling, enemies never get any easier. The earlier Demon Trees don't fall too far behind when you are dozens of levels above the recommended levels, the bosses get more vicious, and the regular enemies deal more damage and have more hit points.
 * Puzzle Quest scales all enemies to the same level as your character, except for boss battles.

Tabletop RPG

 * Over the course of its history, Dungeons & Dragons has been moving in this direction from early editions relying on the DM eyeballing things (or even letting the chips fall where they may using random wandering monster tables) to an increased emphasis on helping Game Masters design properly "balanced" encounters for the party's level as of the formal third edition at the latest. Fourth edition streamlines the process to almost "decide how fast the group needs to advance to the next level, then include that many XPs' worth of challenge" (there's still a bit more to it than that, but it's the basic idea).