Consisting of both a developmental classification and a response-style typology, it clarifies differences and similarities among youths in ways that permit matching of clients with treaters, treatment strategies, and treatment environments. Its central role in California's highly publicized Community Treatment Project resulted in spread of its use within juvenile corrections as well as considerable acclaim and criticism from researchers and policy analysts. This article summarizes I-level's development, classification methods, current status, and future developments. (Publisher abstract)