What is responsible for the differences between the sexes in so many animals, from the brilliant plumage of birds-of-paradise to the antlers on deer? Why are the traits that distinguish the sexes sometimes detrimental to survival? And why do males and females sometimes behave differently? Questions like these have intrigued scientists and the public alike for many years. New discoveries are showing us both how wildly variable the natural world is, and how some basic principles can help explain much of that variation. Like natural selection, sexual selection is a process that results from differential representation of genes in successive generations. Under sexual selection, however, the crucial characteristics that determine whether an individual reproduces depend on sexual competition rather than survival ability. This Very Short Introduction uncovers the history of our understanding of sexual selection, from Darwin's key insights to the modern day. Considering the investment animals place on reproduction, variation in mating systems, sexual conflict, and the origin of sexual dimorphism, Marlene Zuk and Leigh Simmons discuss questions such as whether females can really choose between males on aesthetic grounds, and how sexual conflict is resolved in different species.