The etiology of delinquency has long been a controversial issue, and is particularly so at present. The bulk of the substantive data on which the following material is based was collected in connection with a service-research project in the control of gang delinquency. The major sets of etiological factors adduced to explain delinquency are, in simplified terms, the physiological, the psychodynamic, and the environmental. In the case of "gang" delinquency, the cultural system which exerts the most direct influence on behavior is that of the lower class community itself—a long-established, distinctively patterned tradition with integrity of its own—rather than a so-called "delinquent subculture". The dominant concern over "trouble" involves a distinction of critical importance for the lower class community—that between "law-abiding" and "non-law-abiding" behavior. The one-sex peer group is a highly prevalent and significant structural form in the lower class community.