Throughout the history of juvenile justice, criminologists and others interested in deviant behavior have sought to explain why certain individuals act in certain ways at certain times. The number of theories for deviant behavior has grown considerably over the past 100 years as the field of criminology has progressed and the level of research has improved. This chapter examines different theoretical schools of thought, biological, and psychological, explanations of delinquency and deviant behavior. Biological explanations range from physical appearance theories to modern sociobiological approaches that consider both biological and environmental influences on behavior. Psychological explanations look at psychoanalysis, developmental theories, and attempts to identify antisocial personalities.