About 4% of the world's population admit to having seen a UFO. How is it possible that with more than 200 million known sightings many people continue to refuse to accept the evidence of the UFO phenomenon? Have they really visited us? From the legendary references to an alien presence in humanity's past to recent sightings ... what is the truth? Trying to answer this question, we will review the main evidence and interpretations that we know so far.