We've nearly all been there--awakened with a pounding heart by frightening scenes that seem real, but were conjured up and existed only in the sleeping mind. Nightmares affect people across countries and cultures, with some 10 percent of the world's population reporting recurrent nightmares. Parents have reported, and science has recorded, nightmares in children as young as 18 months. Up to 40 percent of children aged 2 to 12 experience nightmares, as do some 35 percent of veterans and 50 percent of adults with chronic illness. With this book, a psychologist widely known in his field shows how nightmares evolved and were useful to ancestral populations, and why nightmares may carry beneficial functional effects even today for people who suffer from the pulse-pumping dreams. McNamara brings us up to date on the biology of a nightmare and what, specifically, happens in the brain during the event. He also explains the history and development of nightmares and likely causes, including traumatic events, psychological and physical disorders, and commonly consumed medications. Many examples of nightmares are presented and explained. The content of nightmares is given unusually detailed attention, and the latest science on nightmares is succinctly reviewed. Tables in every chapter summarize existing findings and conclusions on nightmares, and strategies for dealing with nightmares are described. In this novel view, McNamara shows why, rather than being harmful, nightmares can be a helpful adaptive system. The special theme of "spirit possession," which frequently occurs in nightmares, is discussed and related to similar themes in horror movies and horror fiction.