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. Spealman and his research team assess the triggers of relapse in nonhuman primates. Jaak Panksepp and associates elucidate the role of emotional systems in addiction via a neuroethological perspective, while Michael T. Bardo and Linda Dwoskin describe the biological connection between novelty and drug-seeking motivational systems. Drive, incentive, and reinforcement, along with factors controlling the reinitiation of drug seeking and the environmental sources of motivation round out the remaining discussions by Roy A. Wise, Jane Stewart, and M. Vogel-Sprott. Rick A.Bevins is an associate professor of psychology at the University of Nebraska-Lincoln. Michael T. Bardo is a professor of psychology at the University of Kentucky.