The Fourth Edition, like earlier editions, provides a broad coverage of the fundamental concepts and principles of psychopharmacology and overviews of majors types of psychoactive drugs. My writing approach emphases connecting basic research findings to the effects we find in humans. In doing so, students not only learn how drugs act on the nervous system to produce pharmacological effects, but they also learn about the techniques used to study psychoactive drugs. Updated editions have come  out about every three years to keep up with this rapidly advancing field. The Fourth Edition reflects a variety of developments including changes in the usage of nicotine and Delta-8-THC, additional clinical applications of psychedelics and gabapentin, and the discovery of new treatments for mental disorders using novel pharmacological approaches. Additionally, it incorporates updated statistics, changes in international law, and new information from DSM-5-TR.