This study, which is based on individual criminal careers over a 60-year period, focuses on the development of criminal behavior. It first examines the impact that life circumstances such as work and marriage have on offending, then tests whether the effects of these circumstances are different for different groups of offenders, and finally examines the extent to which the age-crime relationship at the aggregate level can be explained by age-graded differences in life circumstances. Official data were retrieved for a 4-percent (N=4,615) sample of all individuals whose criminal case was tried in the Netherlands in 1977. Self-report data were derived from a nationally representative survey administered in the Netherlands in 1996 to 2,244 individuals aged 15 years or older. In analyzing this data, we use semi-parametric group-based models. Results indicate that life circumstances substantially influence the chances of criminal behavior, and that the effects of these circumstances on offending differ across offender groups. Age-graded changes in life circumstances, however, explain the aggregate age-crime relationship only to a modest extent.