For more than 50 years, men and women around the world have reported sights in the sky that are strange to their experience and understanding. Inasmuch as many of these sights appeared to be solid objects having impressive performance, they have become known as Unidentified Flying Objects (UFOs) or, colloquially, flying saucers. Their positive identification is one of the major public scientific challenges of the era, with a significant number of these sights so far defying all reasonable efforts at classification.