A criminal career is basically a sequence of offences committed at different ages. It has a beginning, an end and a career length in between. This chapter talks about the prevalence, individual offending frequency, the onset of criminal career research, age of desistance, and offence and practice of criminal careers. Only a certain proportion of each birth cohort commits offences and has a criminal career, and a small fraction of offenders commit a large fraction of all offences. During their careers, offenders commit crimes at a certain rate while they are at risk of offending in the community. For offenders who commit several offences, it is possible to investigate to what extent they specialize in certain types of crimes. It is also important to investigate to what extent crimes are committed alone or with others. The chapter concludes with a brief on the policy implications of criminal career research.