Antigravity is a word with numerous connotations, from its associations with a particular kind of technologically optimistic science fiction that hit a peak of popularity in the middle of the 20th century, to a shorthand symbol for a world-changing scientific breakthrough. The latter context has made antigravity a prime target for fringe theorists, as well as frequently featuring in discussions of the supposed extraterrestrial origin of unidentified flying objects. Do all these speculations presage a genuine scientific revolution, or are they just wishful thinking? That’s the basic question addressed in this book. To lay some preliminary groundwork, this first chapter asks just what is meant by antigravity. This isn’t an easy question to answer, because it doesn’t have a clear-cut scientific definition—beyond being, in some sense, a negation of gravity. This chapter examines what this might entail.