This volume marks the fiftieth anniversary of the Nebraska Symposium on Motivation, the longest continuously running symposium in the field of psychology.

The motivational processes involved in drug abuse, the largest health problem in the United States, are the subject of eight thought-provoking essays that probe behavioral, cognitive, evolutionary, and physiological perspectives. George F. Koob discusses the implications of an allostatic view of motivation in psychopathology. Harriet de Witt considers the dual determinants of drug use in humans, reward and impulsivity, while R. D....