Zebras are infamous for their contrasting black and white stripes and their unique striping pattern among mammals has been discussed since Wallace and Darwin. Many very different ideas have been proposed for why the three species of zebras have stripes but remarkably few attempts have been made to test any of them. This book systematically examines every hypothesis for the functions of striping in zebras using observations principally in the wild but also in captivity, simple field experiments, careful reading of the literature, and sophisticated comparative analyses across equid species and subspecies to reach a conclusion. The hypotheses include camouflage, warning coloration, several different types of predator confusion, ectoparasite avoidance, facilitating social interactions, and thermoregulation. Only one of these hypotheses stands up to close scrutiny: stripes are an adaptation to avoid biting fly attack to which zebras are particularly sensitive. These flies carry diseases that are fatal to zebras. This is a surprising finding in that coloration is animals is usually interpreted as camouflage, warning coloration, signaling, or as a means of heat management so zebra striping provides an important biological exception. The book presents numerous sets of observations and experiments that have never been described before and conducts an extremely thorough review of the literature. It is the most comprehensive book on this intriguing evolutionary mystery to date.