Multisystemic therapy (MST) is a family-based treatment that was originally developed to address serious and violent juvenile offending and that has since been adapted to the treatment of other complex clinical problems in youths. MST interventions target risk factors at multiple levels of youths’ social ecologies (i.e., individual, family, school, peer, neighborhood) and are delivered in the natural contexts in which problems occur (e.g., home and other community settings). This entry provides an overview of the theoretical, empirical, and clinical foundations of the MST model; describes three adaptations of the model: MST for Problem Sexual Behavior (MST-PSB), MST for Child Abuse and Neglect (MST-CAN), and MST-Psychiatric; and provides information about ongoing efforts to disseminate MST and its adaptations into community settings.