The chapter briefly identifies, defines and reviews the empirical evidence of the role of the basic parameters of criminal activity/careers. It includes a review of the theoretical issues and approaches within a developmental perspective on delinquency, highlighting issues of describing population trends versus predicting the behavior of individuals. One section of the chapter focuses on the relative utility of the current leading developmental model, including a test of the model's explanatory utility for explaining serious and violent offending among nationally representative and high-risk samples. It is premature to base policy on the assumption that there is a group of serious and violent offenders who can be readily identified prior to early adolescence and by their involvement in delinquent behavior. However, relatively small sets of pathways can be used to type delinquents and at-risk groups by probable trajectory. The most promising distinction is the relative age of onset and the frequency and seriousness of prior involvement over time. Tables, figure, notes