I like to think they all do, just with Wall*E in the future.
Cars must take place after Wall-E. Obviously their attempts to repopulate the planet didnāt work out so well in the long runā¦ 
Haha, or maybe Cars happened while WallE was alone, and the people got new talking cars, and the repopulation efforts did work out okay. 
Iām stretching it too far, nowā¦either way, I think Up, Toy Story, The Incredibles, Monsters, Inc., WallE, and really all of them work just fine if you have av imagination. Besides, since Vi has a portable DVD player, and I believe The Incredibles IS set in present day, with a 60ās outlook(pretty much what Brad says on both discs) I believe supers were banned in 1989, meaning Andy may have never even heard of them.
These are all very interesting. Thanks for the replies everyone! 
Still, it would be sad fate for Pixar characters in WALL-E. No water for marine life, no laugh power for the monsters, all animals no longer existing. I think the movies are separate and I maintain WALL-E because of live-action.
Hey, maybe somebody should e-mail Pixar or something to see if they are or not.
I think cars is the Car version of the pixar universe, as in, it takes place on a different planet, but there are pixar characters that appear as cars in Cars, (like P.T. Flea).
Not to mention the fact that all but a couple hundred thousand people probably died extremely painful deaths.
Personally, I like to believe that all Pixar films are set in the same universe, even Cars. Iāll explain my views.
A Bugās Life could quite likely be set in the Toy Story universe. The Pizza Planet truck from Toy Story is seen outside of a trailer. This may or may not be where the delivery guy lives. Also, a Pizza Planet Mega Gulp cup can see seen on top of the bug bar before Flik enters it. I think I also remember seeing some of the Root Beer cans as Flik enters the city.
Monsters Inc is also possible. As many know, Randall is banished to the same trailer as the one from A Bugās Life, also with the Pizza Planet truck intact. One of the posters in Sidās Room can briefly be seen in a childās door. Boo also gives Sulley a Jessie doll and a Luxo ball.
Finding Nemo could possibly be set in the Toy Story universe. When Nemo first arrives in the Dentistās office, he looks out of the tank. Here you can briefly see a Buzz Lightyear on the floor in the Dentistās office, along with some other toys. The Pizza Planet truck can be seen as the Tank Gang pictures themselves escaping from the dentistās office. Not to also mention that Mike from Monsters Inc can be seen swimming underwater during the end credits.
The Incredibles is a tough one, since there are very little references to previous Pixar films, possibly because Brad Bird wanted to keep the story his own. For a start, the Pizza Planet truck doesnāt appear in this one. Even Lee Unkrich confirmed this on his Twitter blog. However, the Rock Em Sock Em robots from Toy Story 2 can be seen on one of Mr Incrediblesās shelves if you look closely.
Cars is obviously the most unlikely film to be set in the same universe as previous ones, since everyone is replaced with talking cars and other veichles. However, maybe it could be a parallel world with talking veichles, like the monster world in Monsters Inc.
Ratatouille is likely to be set in the same universe as The Incredibles since both are directed by Brad Bird. Like I said before, he likes to keep his stories his own, and include as little references to previous Pixar films as possible. Unlike Bradās last film however, the Pizza Planet truck does appear in this one. It can be seen very closely on the bridge as Skinner is chasing Remy. Linguini also seems to be wearing boxer shorts with the Incredibles logo on them.
I know I could explain my views on Wall-E and Up, but I think Iāll just leave it at that. Overall, most of them could be set in the same universe, except for Cars obviously.
I personally think exactly like Geoff, with most things in the same universe, and some in parallel universes.
Lets just say that in my personal opinion, all films are set in the same universe. If you have enough imagination, you think there as being several dimensions with different forms of life. One with humans, one with monsters and one with anthropomorphic cars. Thatās the way I see it.
Certainly, although on different planets (Carsā¦), but yeah, the Pixar universe as I call it seems very similar, Up, A Bugās Life, Ratatouille I can see as clearly being on the same planet, movies like Toy Story and Up for example, as well as The Incredibles, the everyday life neighborhoods look very similar in how theyāre shown.
I would say if anything that Monsters Inc. and Toy Story are the same world Y3 there IS a Jessie doll in Booās room YD
Carās could be the same planet as A Bugās Life both have no human interactions.
Wall-e could be a later time period from the world of Toy Story hmmm also Ratatouille could be in the same world as that too.
Hmm but i would think that Ratatouille and Up share the same world rather than Toy Story and Ratatouille aand Monsters Inc.
The main thing to me though is the Dinoco thing (probably spelled that wrong) Its definitly in Toy Story and in Cars. thats a major thing to be shared you get what i mean? YD
Edit
Oh right i forgot about those things from A Bugās Life YD hmm then maybe id say that Toy Story, Monsters Inc., A Bugās Life, and Cars share a world but Monsterās Inc., Cars(with A Bugās Life) are in a parallel dimention YDDD i love we are putting so much thought into this and trying to make it so scientifical (big word 
) YDDDD
Itās also in WALL-E and you spelled it right.
I want them to be in the same āi=universeā, with Wall-E and Cars on other planets.
i dont remember it in Wall-E TT oh no i need to watch it again YDDD thats like my favorite thing to look for!
What if both of those movies are actually apocalypse type movies? What if both are different stories of how human life ended on Earth?
lol, I read an article about the scariest kids movies, and Cars was on it. The author said about Cars: āWhat happened to the humans? We shouldnāt trust our cars anymore! Did we morph into cars?ā 
Eh, I guess so. I just find that really sad and disturbing, even just as a joke. Iād prefer it was on another planet.
I think I got an idea of the timeline and locations of the Pixar Universe.
The Incredibles takes place somewhere in the mid 70ās, after the Golden Superhero Age. The Toys and Cars can come to life, but choose not to around humans, so they donāt need to fend for themselves in an all human world. The reason we donāt see any Cars come to life is because there are too many humans around at every possible chance they get.
So the Monsters use their door-portal tech to transport some sentient Cars into another sentient World, which technology built so they can be used easily.
Also, in case Pixar is still here in a hundred years, WALL-E takes place on a different Earth with the same Universe and same events from past films.
I think probably Finding Nemo and A Bugās Life areā¦ Cars obviously isnāt. Ratatouille, Wall-E and The Incredibles are more dramatic movies, so Iād say those I could take being in the same world seriously. Toy Story, Up and Monsters, Inc. could be.
Up and Ratatouille could go with any though really.
I personally agree with the above. Cars just doesnāt fit into any of the other movies, and Ratatouille doesnāt have too many fantastical elements to it.