im not sure if your serious or not

I thought the only aliens were just foreigners who crossed the border illegally... I must have missed something.

We can not be the only race, humans, in this universe.

There is bound to be other species throughout the cosmos

aliens definitely exist in my opinion, the only question that remains is how many civilizations are there

lots of civilizations, even if you're only looking at earth...

Yea there are a lot of things that we don't know about and they wont tell us either. 

You Aliens that are leasing our planet is proof enough.

Some kind of aliens must excist, how the heck can we be the only alien in universe, no chance to that, somewhere there are som other living creatures and soon perhaps we will discover them.

Ok, let's start by saying that there are no "hidden aliens OMG Conspiracy !!!". Would be too big, would have utterly failed already, the very idea that it could hold in our day and age is ludicrous.

Second, Occam's Razor cut the "ancient aliens" theories down too.

Now, statistics and the sheer size of the universe render almost certain that there has been / will be non-earthly lifeforms, and probably in often enough occurence that some of them would evolve in something more complicated than bacteria.

"Aliens on Earth", intelligent type : statistics and the sheer amount of parameters needed right to get a planet of the good type in the habitable zone of a viable star, with a development range close enough to us so as not to be long dead already, with interstellar travel and our system as an arrival point, all this render the possibility so remote as to be virtually non-existent.

"Aliens on Earth", non-intelligent type : organic compounds from extraterestrial source (current space stations excepted) may have existed on planetoids and teared from there by cataclysms, coming to Earth via asteroids and such. Maybe. Far less costly hypothesis than for the intelligent version, but still very improbable. Universe, size, statistics, all that.

Aliens have been to Earth, I know because my Maths teacher told me so at 15, and again at 16. We got a whole history on it in class, he's even seen a UFO. My Maths teacher was awesome.

Men in black suits are coming for us all

I am half alien

i think aliens exist in my opinion

I have never seen one, saw a spaceship but couldn't see who was flying it.

I really want to believe they do, but surely we'd have found evidence of them by now?

My ex was an alien on earth.  That was the strangest man on earth in fact.  His logic was from outer space somewhere.

I think its possible for alein lifeforms to exist but not in the form that we view in the "movies"