I-level theory is not a theory of delinquency but rather a general theory of personality development. It describes psychological development in all persons in seven successive levels of interpersonal maturity, ranging from the least mature (which resembles the interpersonal interactions of a newborn infant) to an ideal of social maturity rarely, if ever, reached. Each stage is defined by a crucial interpersonal problem that must be resolved before further progress can occur. According to I-level theory, all persons have at any time a relatively consistent way of looking at the world and at themselves. In this study's presentation of I-level concepts and their relevance for offender classification and treatment, integration levels in normal development are first discussed, followed by a delineation of developmental stages and delinquent subtypes among adolescents. Classification methods and the occurrence of I-level groups are then considered along with the implications of the theory for treatment interventions with offenders. The latter discussion focuses on treatment settings, treater style, and treatment modalities. The concluding sections address treatment with specific offender subtypes and the relevance of I-level theory to criminal justice processes and delinquency-causation theories. Seven notes and 40 references are provided.