The main aim of this article is to investigate the duration of criminal careers, to assess how many offenders desist from offending (i.e., terminate offending) up to age 61. The Cambridge Study in Delinquent Development is a prospective longitudinal survey of 411 London males originally studied at age 8 in 1961. Their criminal records have been searched repeatedly up to age 61, and their self-reported offending was measured up to age 48, when 93% of males were interviewed. Forty-four percent of males were convicted, and many criminal careers were very long; 26% of convicted males had a duration of 20 years or more. The probability of being reconvicted was substantial even after a gap of 10 (20%) or 15 years (19%) after the previous conviction, but it was only 8% after a 30-year gap and only 6% after a 40-year gap. Offenders who had been conviction-free for 30 or more years since their last conviction had a similar self-reported offending score to non-offenders at age 48. It seems likely that those offenders (49%) who had been conviction-free for 30 or more years had truly desisted. In contrast, those offenders who were conviction-free for less than 10 years (22%) probably had not desisted up to age 61. Desistance is more uncertain for the remainder of the offenders.