The word ‘UFO’ is an acronym for Unidentified Flying Objects. In other words, it is a flying object whose nature is not known. Weather balloons, certain types of clouds, high-flying aircraft, rocket stages, and the planet Venus have been often mistaken as UFOs. In common parlance, the term UFO has, however, come to mean an alien spacecraft. Also, there are some puzzling events and observations which have led to the speculation that some UFO sightings indicate the visits of alien or extraterrestrial beings from another world to earth. However, there is no technical evidence found to date which substantiates the belief. Astronomers are nevertheless continuing their search for extraterrestrial life in the universe.