Although no one can prove it one way or another, unless that person is Chris Nolan, it is hard to say. Both theories are possible.



Evidence to Reality: If we accept that the entire movie is not a dream, and that Cobb did in fact escape his limbo with Mal, then it is reasonable to believe he also escaped his limbo with Saito. Also, there are simple explanations as to why when he sees his kids again they look much like they did in his dream. For example, he never saw his kids faces so we cannot easily tell if they had aged much from his memory. In addition, the movie never defines the amount of time that has passed since Mal had died and Cobb was banned from the U.S. So the memory of his kids may only be a few months old, therefore it is very possible his kids look pretty much the same in the memories and in the present. In addition, the visual duration of the top spinning is never a trustworthy totem at all, since it is a property of the totem others can observe and recreate. If the top is to be trustworthy at all, it is Cobb's knowledge of how it feels to spin it that convinces him he is awake, not the duration of the spinning, which we can presume will end eventually in the final scene. Visually, there is an obvious difference between the wobbly spinning in the end-scene and the absolutely perfect spin in the safe during limbo, again implying the possibility that the final spin could be natural (though only the feel in Cobb's fingers would know for sure). It is alleged that at the end of the film Cobb's children are wearing different, albeit similar clothing compared to the clothes they wear in Cobb's subconscious. In the opening scene dream, you get a glimpse of Cobb's hand. He's wearing his wedding ring. Now, if you follow the rest of the movie keeping an eye out for this you will notice that he only has the ring on when he's allegedly in a dream. At the end of the movie he isn't wearing the ring. If the ring only appears when he's in a dream and he's not wearing at the end of the film, that could be confirmation that in fact he woke up. The sound of the top falling over is played after the credits indicating that it is indeed "reality", but the important part is that Cobb is not looking at it, because as Nolan says, "He doesn't care."



Evidence to Dream: It is possible that the entire movie is a dream, and that Cobb never left his own limbo. The children appear the same age as in a previous memory, and in fact, appear to have the same motions and same clothes in the yard before turning around. It suggests to the viewer that this is exactly the same as his memory, and thus he could be still dreaming. How long the top spins is not a good totem, except in that it spinning forever is only possible in the dream world. A short-duration spin is a visual property that someone could easily recreate in a dream. If we trust the spinning top, it must be the exact feel in Cobb's fingers as he spins it that convinces him it is genuine. However, Cobb admits that the totem was originally Mal's totem, suggesting that he adopted it as his totem during limbo, making it impossible for him to know the true feel. In addition, a totem can not prove that you've woken up from your own dream, since you know it's true "feel" and thus your subconscious can recreate it properly.



Also: This may be a continuity error, but when Arthur comes to get Cobb in Tokyo, they leave the hotel room and go to the roof for the helicopter at night. When they're on the roof, it's day.



The "both theories are true" conjecture: Movie has two main themes: the first one is that the line between dreams and reality can be imperceptible from the perspective of an individual; the second that an idea can be a parasite which, once incepted/inceived, can take over one's psyche. The ending simply reflects the fact that the idea that killed Mal has now taken over Cobb; he cannot discern whether he is now living in a dream or a reality, and we, the audience, witness that intimately. The gyro in the end falters, but we do not see whether it stops or not. Have we witnessed it stop, the movie would be essentially telling us "fear not, this is real and a happy end". Had the shot ended with it spinning, the movie would affirm the other theory. However, the shot as-is wants to ensure us that we do not know and we cannot know, because Cobb doesn't know anymore. The lack of definite answer IS the answer; for the rest of his life Cobb will wonder whether everything around him is real, and the audience is deliberately left with that same question hanging in their heads. Notice that when Cobb is in his "Memory bank" with Ariadne, Cobb is looking out the window at his kids, he tells Ariadne that his greatest regret is not being able to alter this moment and see his children's beautiful faces again. That would imply that was Cobb's inception to himself. He finally did what he could never do before and altered the moment in which he saw his children and he sees their faces. Also, why was Miles there? He was living in France yet was waiting at the airport in the States (reason why MIles was there: When Cobb visited Miles it was to give him a gift for his children, i.e. Miles was traveling to see Cobb's children in the near future, and obviously Cobb could and would have made arrangements with Miles when he knew he was coming back to America). Even more confusing, Miles' wife, Mal's mother was taking care of Cobb's children in America yet we never see her at the house at the end . The inception itself could be altering his biggest regrets which spreads to every aspect of his life he winds up needing to alter. Or Mal was right the whole time and he was actually still down one dream level and Mal is alive in reality and Cobb wasn't waking up.



Unknown group trapping Cobb explanation: Cobb is the best in consciously dreaming, so he'd not use the spinning top as his token if it wasn't reliable. The whole idea in it is that IRL the top will lose energy and fall, while in a dream it can spin endlessly. When Mal created this technique it meant to help figure if they were dreaming or awake, as when (an untrained) one's asleep he doesn't note he's sleeping. But for thieves, it's used to assure they aren't being manipulated by somebody in a dream. During the movie, for 2 times Cobb tests the top and it shows he's awake, then he goes test it but it falls on the ground and Saito enters the bathroom and he's unable to test it again. Then he, disobeying himself, tells Ariadne how the top works, and then only at the end of the movie he tries it again. In this theory, sometime between the 2nd and the 3rd test he's locked in a dream, by some unknown group. On the 3rd test the architect makes the top fall so he doesn't test it, while the architect still doesn't know how it works. Ariadne is in fact an agent, who seduces Cobb into getting his trust, and then he tells her how the top works. She then goes analysing his psychology deeper and deeper, looking for his weak point. At first she discovers about Mal and it seems she's his weak point, that he wanna lock himself on an eternal dream to live with her, but then she notice he's always rejecting her and that Mal isn't good enough. They go on the whole inception project, which in fact is just a plot to mess with Cobb and make him lost on what's a dream and what's reality, it's all very tricky to take his attention into that and make him even more messed. Until he finally succeeds and is taken to see his children. He then tests the top again, the architect makes it move around to look like it's gonna fall, and he's immediately called attention to his children. During the movie he never allows himself look on their faces because he know that's gonna make him fall, but when he thinks the top stopped he lowers his defences and accept to look at them, and when he sees their faces he finally falls and doesn't even bother to take the top from the desk, because all he can think of is them. But in fact he's still in a dream and trapped.



Yet for an inception to work, one must accept it as one's own thought process... so of course Nolan does not give interviews where he says, "This is a movie where I'm trying to wake people up to the fact that they're dreaming." Edit