Beginning with a group of approximately 10,000 boys born in 1945 who lived in Philadelphia from at least ages ten through seventeen, the Center for Studies in Criminology and Criminal Law, University of Pennsylvania has engaged in a longitudinal analysis of the delinquency of the birth cohort. The first publication which examines the dynamic flow of delinquency committed by 3500 of the boys was published by the University of Chicago Press in 1972 and is entitled Delinquency in a Birth Cohort. This study is the first in the United States to establish a base line of delinquency probabilities and to analyze the types of delinquency committed over time, with the recording of the seriousness of each of the 10,000 acts committed by the 3500 boys. The seriousness scores were derived from the earlier work by Sellin and Wolfgang, entitle The Measurement of Delinquency. A stochastic model was used to analyze the delinquent patterns and one of the major conclusions, at least up to age eighteen, was that there was no specific delinquency specialization by type of offense. Moreover, after the third offense, the probabilities of desistence, or refraining from further delinquent acts, remained stable, thus indicating that the most propitious point for social intervention would be after the third offense rather than at some time prior to the onset of delinquency or even after the first or second offense-offenses which are usually of a very minor character. The Center for Studies in Criminology and Criminal Law has continued to follow up a ten per cent sample of the original birth cohort by interviewing them to obtain additional social psychological dynamic features of their background, of the situations involving their first and last delinquencies, and of their adult careers. The follow-up indicates thus far that approximately 12 per cent new cases of criminality appeared, thus adding to the original 35 per cent of the birth cohort who had a delinquency record. The study will pursue data analysis of the elaborate interview schedule and a continued examination of recidivism, particularly chronic, among the chronic offenders who have committed five or more offenses.