The Cambridge Study in Delinquent Development is a prospective longitudinal survey of a sample of 411 males. Data collection began in 1961-62 when most of the boys were aged 8 and ended in 1980 when the youngest was aged 25 1/2. At the time of first contact, the boys were all living in a working class area of London, England. Only 12 of the sample were black. The subjects were interviewed and tested in their schools at the ages of 8, 10, and 14 and were interviewed in the research office at the ages of 16, 18, 21, and 24. It was always possible to trace and interview a high proportion of the original sample. At age 21, the aim was to interview only the convicted delinquents and a similarly sized, randomly chosen sample of unconvicted youths. The peak age for most offenses was found to be within a year or two of the 17th birthday, although shoplifting and stealing from machines seem to peak earlier and fraud later. There was also found to be a close relationship between juvenile and adult delinquency, with the youths convicted at the earliest ages (10-12) being the most persistent offenders. The probability of one conviction following another reaches a peak of about .90 for youths with six or more convictions. The fact that self-reported and official delinquency tend to peak at about the same age suggests a real peak at about age 17. The results are compared with other studies, and some suggestions are offered to explain the peaking of delinquent behavior at age 17. Tabular data and 19 references are provided.