Dexter's Laboratory/WMG

Regarding the show's incestuous subtext.
Even for an older sister, Dee Dee is rather...attached to her baby brother. She's very touchy-feely, tries to distract Dexter from his work all the time, and gets upset when Dexter ignores her. He's the only character she treats this way, and it evokes a girl who's too into her love interest.

The creators of the show probably wanted a kind of reclusive guy, extrovert girl plot...except that Dexter would be too aloof and preoccupied to catch on to her advances.

In the end, they decided to just make the two siblings, probably to explain how she's always at his house if he doesn't like her much. But they kept the trappings of a one-sided romance.

DeeDee is Eris from The Grim Adventures of Billy and Mandy.
A younger form? An avatar? A daughter? A herald? Whatever she is, there's nothing better to disrupt the delicate machinations of science and technology than the divine powers of chaos and discord. If she grows up to be a total babe with an accent as inexplicable as her brother's, well... no surprise here.

DeeDee is not Dexter's sister, but his future daughter.
Some time in the future, Dexter has a daughter but also creates an invention which threatens the world. DeeDee is sent back in time to prevent this from happening by pretending to be his sister and destroying his inventions that could set up her future. She can get into Dexter's lab no matter what protections he puts on it because, to her, they've always been there; she knows how to bypass them. And DeeDee is a codename, literally standing for 'Dexter's Daughter'.
 * Ego trip may be the idea that Dexter in the future finds out about her plans and wants to stop her. When the robots are sent, Dexter destroys them, and then the rest of the plot follows up to the other Dexter's sending their own wave after Dee Dee.
 * Alternatively, they're opposite sex clones. Either she's from the future, or Deedee used to be a supergenius, but in the experiment that produced Dexter, her advanced knowledge got transferred to him, and she became a massive ditz). She's able to bypass the security features because, as far as the security features know, she is him.
 * As far as we know, Dexter could be some sort of technological Haruhi, and Dee Dee is there to stop him. Except that he won't grope her. Eww.
 * Deedee's Dexter's granddaughter; that's why she called him gramps when he was old in that episode. It's because he is her grandfather.
 * Like Fry, Dexter is his grandfather due to time travel.

Dexter is Dexter from Dexter.
What? He hates people, is super smart, and has an oblivious sister. It makes everything slightly less confusing.
 * I like this one, but it is immediately Jossed by the fact that DeeDee knows about the lab, while Deborah knows nothing of Dexters "hobbies". can this hole in the theory be patched?
 * So, wait, does that mean Mandark dismembers hookers?
 * His parents named him Susan, who's surprised?
 * Alternatively he buries boys, slices young women in bathtubs, throws family mothers from bridges and blunts family fathers to death with hammers. In this order.
 * Dexter has red hair, trouble fitting in with normal people, above average intelligence, talks to himself constantly, and clearly feels some level of disgust with the world around him.
 * Perhaps the "secret lab" under his house is really just young dexter's way of referring to the basement where he butchers animals.
 * Dee-Dee is obviously a nickname, short for Deborah, Dexter's childish and easily-excitable sister.
 * As for Dexter's taller, thinner, dark haired evil counterpart, that would have to be Mandark. Obviously no young man would want to go through life with the name Susan, so he switches to Brian later in life.
 * Dexter's accent is obviously an affectation, as no members of his family share this accent. Presumably he drops it once he begins to mature. The show Dexter's Lab is set before Dexter's 'dad', Harry, realises exactly what his son has been doing down in the basement.
 * Continuing on from the above troper's guess, Dexter's Lab is seen from Dexter's point of view as a young child, hence the cartoony style, lightheartedness, and no hint of Harry not being Dexter's real dad.

Dexter's Lab is the prequel to The Big Bang Theory.
This says it all

Dexter is a leprechaun, or a Half-Leprechaun.
He's short, red-haired, and does things that defy the laws of physics. Oh sure, he calls it "science," but he never once employs the scientific method. His uncle uses real magic and is clearly Irish.
 * In Ego Trip, his future self is a giant, which kind of josses this.
 * Tall leprechauns have been seen in numerous settings. Joss denied.
 * Like in The Life and Times of Juniper Lee.

Deedee's smarter than she's letting on.

 * It could be there is no wild conspiracy beyond Deedee just being smart. This troper always has believed that was the case.
 * She's never seemed dumb by any stretch, just playful and hyperactive. It takes a level of brains to regularly bust into Dexter's lab.
 * Agreed. Recall that she mastered general relativity before Dexter himself did.
 * Then that means Dexter didn't make her smart in the science project episode, he just removed her hyperactivity so she can focus her intelligence.

Deedee is cosmically aware.
This kinda makes her a bit of a jerkass because she knows exactly what Dexter or anyone else means or is trying to do and still goes out of her way to be the ditz.

Dexter is a Son of Ether.
Seriously? Why is this even a theory. Unless magic was involved, I seriously doubt any of the stuff Dexter builds could have been built by a 8-10 year old.
 * More likely he's a Neid Artificer or Atomist, quite possibly an Unmada or even Illuminated.

The nut Dexter's often seen turning indefinitely is a wind-up system to power up his lab.
He designed a huge, complicated power generator that would be able to provide days' worth of energy for his gigantic lab by expanding the energy generated by that simple turning motion.

Dexter becomes the Professor from Jump Start 3rd Grade
And Polly is his daughter. They do kind of look alike...

Dexter's family name is Dexter
This is why they call themselves the Dexter family.
 * So, you're saying Dexter's full name is Dexter Dexter? Or Double D for sho- My god.

Dexter is just playing around in the basement.
He's a gifted kid with a great imagination. His lab is really comprised of cardboard boxes, Legos, Kinnex, and Tinker Toys, with the occasional science kit thrown in. Dee Dee plays along with his daydreams about his "lab", but the real reason he hates her is because she's constantly breaking his toys and reminding him of reality. His parents are none-the-wiser of his lab because they know it doesn't exist. He and Mandark are actually friends with a pretend rivalry, and the Dial M for Monkey and Justice Friends segments are the worlds he's created with his action figures.
 * We've seen him in the Powerpuff Girls via cameo, at Pokey Oats. Not to mention Major Man exists in both shows.

Dexter is living in a projection of his insecurities.
Dexter, in reality, gets terrible grades in school and is not very bright. He envies the students that do well, and has grown jealous of them. This is why much of what he says is random technobabble, the amount of Cow Tools in his lab, and why none of his experiments seem to have any purpose other than BEING "scientific things": It's all in his head. Dee Dee is actually an Honors student, and because of Dexter's jealousy, she appears in his fantasies as an idiot. Mandark is the head of Dexter's class, and as such, gets the role of Dexter's rival. Mandark's lab is what Dexter really imagines him to have.

Dexter goes on to work at Black Mesa
After moving out of his house to perform his experiments away from Dee-Dee's meddling, Dexter realizes he has to change his name to escape the stigma attached to his old one that came about as a result of his lab being exposed to the public.

He changes his name to Gordon, after his favorite show, Flash Gordon, and changes his last name to Freeman because he will finally be a Free Man. To further mask his appearance, he also dies his hair a darker shade of red/Brown and grows a goatee.

Using money made from DoD contracts at his existing lab, he attends MIT (Being unable to attend harvard for reasons that would later become a Mystery to everyone, even those involved.

After Graduating, he moves to New Mexico and seeks out occupation, eventually settling for a Laboratory not unlike the one he owned as a child...

The rest is history.

Dexter is Calvin.
Except he's an alternate universe version, in which he embraced science instead of just his imagination. Think about it: both of them are very intelligent children, and are highly imaginative - it takes some serious imagination to invent some of the stuff Dexter creates. Calvin invented a thing or two, as well (the Transmogrifier, for example) but lacked the resources to make it truly real. They both have non-human companions who they trust and look to for advice above anyone else (Hobbes and the Computer). And, both Calvin's and Dexter's parents are nigh oblivious about what their sons get up to when they're alone.

Mandark moves to Dimmsdale
After turning his mom from a hippie to a gangster wannabe, Mandark decided he wold punish himself. He moved to a small town named Dimmsdale. He changed his name to Denzel because no adult man wold want to go through life with the name Susan. And he chaged his last name too Crocker because his old last name Astronominov was too hard to pronounce. Then he went to work at Dimmsdale Elementary School. Then he gets ready for a life unlike the one he had as a 16 year old.

Dexter has Aspergers Syndrome
This troper has it, and she sees aspects of it in him.
 * The previous editor didn't elaborate much on this, but it actually makes a lot of sense the more you think about it. In fact, it could be that Dex has Asperger's or some sort of low to mid level Autism and the whole show was actually from his perspective. The characterization is all true, but everyone is suffering from characterexaggeration because that's how Dexter thinks of them. Suddenly, some of the shows quirks seem more resonable:

1. Dee Dee characterization switching from a hyper active little girl to a elitist snob (watch any episode about fashion.) is because he just doesn't get girls. This is probably why his most reoccurring bullies were a pair of girls.

2. His Mom isn't actually that big of a moodswinger, Dexter just doesn't know when he's pissing her off.

3. His Dad just HAS to be less bumbling than that.

4. Here's the most important point: Mandark. Notice how different the characterization was between the beginning of the series and the end of the series. (Of the Mc Cracken run.) Let's remember the first episode he was in; this was the first time he met a kid that was on his IQ level. Throughout the entire show's run, Dexter was shown to be hyper-competitive at whatever he did. So no matter what this guy was like there was no way Dex was going to like him, so who knows how "accurate" his characterization in that episode was to the actual Mandark. We even get hints that Dexter is mentally blowing things out of proportion by how the entire school faculty acts in that episode. In the beginning of the episode they were figuratively worshiping him, by the end of it they were absolutely starstruck by Mandark and LITERALLY scorned Dexter because the new kid was smarter than him. Think about that; PUBLIC SCHOOL TEACHERS, worshiping a STUDENT, for answering questions. If you don't think that's odd, you've never been to school. Period. Then what's curiouser is that after that episode, Mandark stops outshining Dexter. Suddenly, they seem to be on the exact same level mentally. Even odder, Mandark gains quite a few pathetic qualities to him (Gets bullied by his little sister, jealous of Dexter, can't get over Dee Dee, never wins a battle since, etc.) Things get to the tipping point when Mandark get his dayinthelimelight episode where he tries to win a surfing competition to win Dee Dee's heart. In that episode, if it wasn't for the fact he was swooning over Dee Dee, Dexter could have easily been the protagonist and you'd never even blink. If I had to harbor a guess, whoever is the main character of an episode, the story is being to from there eyes. (Which explains a lot of "Mom"'s kung fu scenes. -Felix Martinez
 * Don't forget that he also shows traits of being socially inept, has very narrow and specific focuses (i.e. his latest invention), often takes speech literally, etc... need I go on?

Dexter and Mandark both have a vague mental disorder that's connected to their intelligence.
I can't think of a another reason why else these two boys sound vaguely foreign but have American parents. With Dexter, he sounds like a kid doing a bad/childish Austrian accent, while Mandark's could really be anything. Hell, I thought Mandark actually WAS foreign, with him looking Chinese(ish) while his sister had some sort of Mongolian look to her. This is completely thrown out the window when we finally see their parents and they happen to be completely white. (Seemingly, anyways.) It could be that Craig McCracken intended for him to be asian, but the new production team had different ideas. Whatever idea the case maybe, it reminds me of this disorder: http://en.wikipedia.org/wiki/Foreign_accent_syndrome -Felix Martinez

Most of the episodes of the series take place in a simulation designed by Dexter to determine everything that might go wrong with his laboratory before he gets too far into building it.
This explains the Negative Continuity as well as why Dexter, who's supposed to be a genius, keeps messing up. He's doing it on purpose to test his future lab.

Mandark is a Hero Antagonist
We're just seeing things from Dexter's point of view.
 * "Sun, Surf and Science" is focused completely on Mandark, and he's shown in a more sympathetic light than usual.

"Dee-Dee's Room" is just a nightmare of Dexter's
That episode is surreal even by Dexter's Lab standards.

Dexter moved up a grade or two
Everybody in his class looks older than him, and he is a super-genius after all.
 * That also explains why Mandark (who looks older than Dee-Dee) is in the same class as him.