Aliens are beings that did not originate on planet Earth but are from another part of the universe. The majority of aliens that are discussed are usually advanced species - intelligent, scientific, and with superior technology than human beings.

Popular claims involve alien visitation on earth, abductions, ufo sightings, ancient astronauts and other paranormal phenomena. Several people have claimed to have witnessed an alien visitation and some people believe they been abducted be aliens and experimented upon. The common depiction of the extraterrestrial is the thin, formless body with a big oval green head and slanting eyes although new and creative examples are surfacing all the time.

Claims about aliens are extremely popular in our 21st Century - movies, books, video games and tabloid headlines often explore the subject. Their popularity has increased since the Roswell Incident in 1945. Claims associated with aliens are often combined with conspiracy theories - such as the government is intentionally withholding the truth about alien visitation and activity on Earth and that captive aliens are kept in top secret locales throughout the world.

Considering the vast expanse of our universe it is considered acceptable through the probability equation known as "Drake's Equation" to believe that life exists elsewhere in the universe. This claim, however, is often confused with the claim that intelligent life-forms have visited Earth. The two claims are not synonymous.

We have explored our own solar system to such an extent that we can reasonably conclude no advanced civilizations exist. The closest possibility is 4 light years away in the Andromeda Galaxy. Light takes 40,000 years to travel this distance and objects with mass take even longer. If alien visitation was possible, the aliens would either have to live on the spaceship for many generations, have excessively long life spans, or have technology capable of defying the laws of physics. Other possibilities have been devised but still, the final decision is made based on evidence. No tangible evidence is available to support the alien visitation claims nor that aliens exist. No radar or satellite has picked up an anomaly in space that could clearly be defined as a flying saucer, or messages and technological signals from an alien civilization.

The variety of phenomena often included under the auspices of aliens and ufos have proved to be other objects and experiences. The personal experiences and eyewitness testimonies that are recounted in cases of sightings and abductions often prove to have alternate explanations. A long list of mind and emotional phenomena provide some information for the claims. The unreliability of memory, hallucinations, and personal trauma are possible factors involved in the experiences recounted.