Can evolutionary theory explain depression, anxiety, and other mental disorders? The authors re-examine this old question in light of current research and show that evolution may provide the essential framework for understanding both everyday human behavior and a range of mental disorders. Their discussion includes up-to-date research on emotions, moods, symptoms, and mental processing. The authors make a compelling case for the view that prominent features of mental disorders are simply adaptive responses to the environment and life's circumstances and that these responses can only be understood in the context of our long evolutionary past.