Criminological psychology strives to explain the criminal behavior of individuals and is also concerned with reducing and preventing criminal behavior. Two of the main questions for criminological psychologists to address are: (1) why do people become offenders, and (2) why do people commit offenses? The distinction between these two questions is that the first tries to explain the development of offenders (between-individual differences) while the second tries to explain the commission of offenses (within-individual differences). The author suggests one of the best ways to compare between-individual differences and within-individual differences is to carry out prospective longitudinal studies with frequent data collection opportunities. Additionally, more knowledge is needed about individual and family risk factors for offending, as well as biological and community risk factors and protective factors. Experimental and quasi-experimental studies can help identify causal factors in offending behavior, as well as interactive factors that affect individual offending patterns. A focus on situational influences, as well as decisionmaking and learning processes, will help address the question of why crimes are committed. Overall, criminological psychologists should focus on longitudinal and experimental studies that will provide the evidence needed to develop and test comprehensive theories of individual criminal behavior. Figures, references