The authors argue that research which focuses on heterogeneity and the initial transition stages of criminality can provide insights into behavioral dynamics previously overlooked in the study of the onset and development of criminal careers. Using computerized police archival data from a large western Canadian city, the study used event history survival models to estimate patterns of onset and persistence for delinquent careers over 6-month intervals in the early stages of police contact. The results provide strong evidence of meaningful short-term change in offending among multiple levels of youth subgroups. Some youths apparently offend only briefly and then desist, and others continue offending and at a higher rate; however, there also appear to be patterns of heterogeneity within as well as between these subgroups of young offenders during the early stages of criminal careers. This paper concludes by identifying legal and social contingencies that may potentially influence events and turning points that constitute the processes common to criminal careers. 5 tables and 69 references