It is important to explain between-individual differences in the development of criminal potential and within-individual differences in the commission of crimes. More research is needed on protective factors, on investigating which risk factors are causes, and on causal mechanisms linking risk factors and offending. Questions about situational influences on offending should be included in longitudinal studies of the development of offenders. Comprehensive theories of offending need to be developed and tested. More longitudinal and experimental studies need to be carried out by criminological psychologists.