The authors review the history and current role of “assertive community treatment” (ACT) in managing complex clinical populations. They trace the origins of the model from its beginning in Madison, Wisconsin, and review empirical evidence of its effectiveness and dissemination to date. Adaptations to treat specific subpopulations such as the homeless, individuals with cooccurring substance abuse problems, and those in rural areas, are discussed. The authors also describe adaptations to provide specific additional benefits. The article concludes with a discussion of efforts to identify the critical elements of ACT programs and to measure the fidelity to the model in the implementation of such programs.