UFO's, or unidentified flying objects, are actually very common. If you see anything in the air, that you can't identify, it's a UFO, and remains one until it has been positivly identified. . .
However, if you're refering to those so-called alien space shuttles, flying saucers, or whatever. . . I do not believe in that nonsense.
Although I don't believe in them, that does not mean I don't believe in aliens. . .
I do believe that aliens exist. Not in this galaxy, or even this super-galaxy (a cluster of galaxies), nor even in this Universe probably (yes, I also believe that there are more than just one Universe -- No, not in terms of Jet Li's "The One"). I do believe that there are other living, thinking lifeforms outside of Earth. Whether they be "beings" or just mere "animals" I do not know. I don't believe they have saucers that can fly at "warp speed" or any of that nonsense. In fact, I don't believe they are any more advanced than us, if not only slightly.
I believe that if they have achieved space exploration, however, all of that Star Wars/Star Trek kinda stuff is just bullshit. I've always belived that the idea of a flying colony, where in that colony, they produce their supply of fuel, reproduce themselves and train their children to fly the ship/colony, in a neverending serach for other intelligent life.
But that's just me.