Certainly aliens, say extraterrestrial sentient human-like lifeforms do exist, if not in this, then in other universes, dimensions, realities and space-time continuums plus extensions, I'm quite sure about it, as part of my beliefs. If they, in fact, did visit Earth somewhere in the past or are right here in this moment somewhere on the planet, is another question. I have no guess.
I'm a science fiction fan and into conspiracy theories.
Most classic "alien abduction" stories are result of "sleep paralysis", a state when you're half-awake half-asleep eyes open but muscles paralyzed. Your mind tries to interprete things like people (humans, for us) and certain objects into shadows, light casts and objects inside the room. Those "Grey" aliens are just mind-created images of simplified humans, the bed becomes the obduction plate, dim light create the spaceships white sterile environment.
According to conspiracy theories, UFOlogy and esotericism dozens of alien races have visited Earth by now, Anunnaki, Reptilians, Greys, Nordics, Sirians, Aldebarans, Formalhautans, Centauri, Pleiades, google it if you're interested. Many are part of the Star Nations organization, something like our UN in space and the "Galactic Confederation", an interstellar alliance formed 4 million years ago in defense against interdimensional invaders.
The Anunnaki from Nibiru, the 12th planet of our solar system (Pluto, the Moon and Sun are the lacking three), created humanity out of other races like the Reptilians and Greys in ancient Mesopotamia were they were worshipped as gods by the Sumereans. In another version, they manipulated evolution over millions of years.
The Sirius system (20 ly away from Earth) is home to a giant being of higher consciousness. The Anunnaki later wiped out human civilization by destroying Atlantis and Lemuria, so they helped the US government to do the 9/11 inside job because otherwise the Anunnaki would have had to stand trial before this higher consciousness.
Other alien races are flying to Earth all the time, just travelling here over hundreds of light years to fly around in the atmosphere or do experiments on some cattles.
By now Earth has a secret spacefleet entitled "Solar Warden", too. Moon, Mars and some gas giant moons have been terraformed in the future and the Solar Warden human and alien crews are space-time jumping to them.
Er... no, that's all crazy conspiracy nonsense, I wonder who believes this stuff? ^^
Now serious: There are ~200 billion stars in the Milky Way galaxy, with ~100-200 billion planets plus billions of large moon, of which around 500-1000 million can lie within habitable zones. If half of them lies there and every 100th is habitable (naturally habitable with native species, settled and/or terraformed) that would leave us 2.5 to 7.5 million habitable planets and moons. For an example, it could be any other number. Some scientists assume about 100 million planets and moons, which seems to fit in my eyes, if considering terraforming and astroengineering (move planets inside habitable zones, turn gas giants into protostars, take life to the planet, build a breathable atmosphere, shape continents and oceans, get enough water from other planets and asteroids, use orbital mirrors to heat it up or cool down, move moons to the planet and so on). Calculations of a billion celestial objects habitable are a bit to high in my opinion.
I have no guess how life can come into existence on other planets and how common sentient life is. I believe in evolution/intelligent design, as it is likely life and sentience evolve on a lot of planets if there are so many while I don't see a reason for so many habitats existing without inhabitants, I believe it's likely that quite a lot of sentient races exist. If you ask me about it, ~100-500 intelligent space-faring even a bit human-like alien species can have come into existence in the Milky Way galaxy.
To come here, they have to possess interstellar spacetravel technology.
Faster-than-light can be possible, by travelled to an extension of our dimension like a hypothetical "hyperspace" with different structure/natural laws/distances/plane of existence, creating rips in the fabric of space-time, warping space around a starship, manipulating a spacecrafts's mass/matter and it's nature and using something with "quantums".
Otherwise, by sublight travel a species could colonize it's entire home galayx in a matter of 10 million years.
I forgot to mention, if aliens haven't visited Earth yet, the "Fermi paradoxon" can be a serious question, about "If there are so many alien races, where are they?". I don't know, maybe there are no ones at all, no aliens in our stellar neighbarhood, they haven't found us, don't possess the technology or hide from us with reason? Maybe an anomaly in space/the galaxy? So many possibilities why they have not colonized life-supporting Earth long before humanity evolved/was created here. Or they have we're part alien ourselves?
The truth is somewhere out there or to quote the Sacred Scrolls of Pythia from Battlestar Galactica: "Life here began out there."
@BronyPony You should have made another option in the poll "I don't know" or "I believe yes or no", because, how can we be certain that they in fact have visited Earth or have not?
@Topic If to choose I'll say yes, it's just a bit more likely than visitors from other stars never walked on Earth.
Space Warrior