Both humans and animals live in a rich world of events. Some events repeat themselves, whereas others constantly change. The authors propose that discriminating this stability, sameness, and uniformity from change, differentness, and diversity is fundamental to adaptive action. Evidence from many areas of behavioral science indicates that the discrimination of and preference for stimulus variability affects both human and animal action. Recent comparative research with humans and animals illustrates a promising approach to the study of these issues. Discovering and understanding the behavioral and neural processes related to stimulus variability and its consequences for behavior offer distinctive challenges and important new opportunities for psychologists and neuroscientists.