Katherine Teilmann Van Dusen and Sarnoff A. Mednick This introduction delineates what we consider to be three of the most important impediments to the advance of knowledge in the field of criminology. The most fundamental need is for more studies of the nature and progress of criminal and delinquent careers. The second need is for more prospective, longitudinal studies of the etiology of crime and delinquency. The third need concerns the lack of interdisciplinary research toward a more integrated understanding of delinquent and criminal behavior. Criminal and Delinquent Careers The birth cohort study by Wolfgang, Figlio and Sellin (1972) was heralded by many (Farrington, 1973; Erickson, 1973; Weis, 1974) as a landmark which allowed researchers to study the course of delinquency without the usual sampling biases that plagued other, cross-sectional research. For the first time, we could get a reasonable picture of when delinquency usually starts, what proportion of the population engages in delinquency, what types of delinquencies they engage in, what proportion continue, and so on. Cross sectional studies do not permit the investigation of careers because cross 1 PROSPECTIVE STUDIES OF CRIME AND DELINQUENCY 2 sectional sampling includes only portions of careers for many of the individuals sampled. This is just one of the many problems that restricted researchers' ability to study the nature of criminal careers.