This paper presents the main findings of quantitative and qualitative research into the criminal careers of about 1000 offenders who were involved in 80 extensively analysed cases of organized crime. The paper analyses how and when offenders become known to the criminal justice authorities, studies in depth the criminal careers of 'starters' and analyses in detail the criminal careers of (ring)leaders and 'nodal' offenders. Because social ties play an important role in organized crime, the paper emphasizes that the social opportunity structure, defined as social ties providing access to profitable criminal opportunities, is extremely important for explaining involvement in organized crime. It explains why certain offenders 'progress' to certain types of organized crime whereas others become involved only later on in life. Social opportunity structure may also explain interesting phenomena such as 'late starters' - people without any appreciable criminal history - and people in conventional jobs who switch careers.