The overall aim of this R34 application is to develop an adaptation of the strongest evidence based program to reduce juvenile offending, Multisystemic Therapy (MST), for offending reduction and increased role functioning in transition age youth (ages 18-25) receiving intensive public mental health services. Avoiding negative civic functioning, namely criminal offending is a cornerstone of adult functioning. Recent findings suggest that the majority of transition age youth with serious mental health conditions will be arrested by age 25, most with multiple arrests, and for serious charges. Because of the serious implications such offending poses for youths, their families, and society, an intervention that proved effective in limiting offending among members of this population would contribute substantially to the public good. To develop such an intervention, adaptations to MST are drawn from the literature of malleable causes of offending and desistance in adolescents and adults, with and without mental health conditions. The proposed project will specify and refine the clinical approach, protocols, and fidelity process, and determine the feasibility of the treatment and research protocols through a 4 stage course in which the first stage (months 1-2) obtains written input from national experts, and through focus groups of providers, youth, and family members on the proposed clinical adaptations. The second phase (months 3 to 7) will consist of development cycles of the MST-TAY program. In each cycle, we will draft a version of the intervention, test it with 5-10 clinical cases, obtain feedback from clients and MST clinicians, then generate a new version of the intervention. In the third phase (months 8 to 30), we will conduct a larger pilot for the purpose of determining feasibility of the research approach, using the stable, final version of the intervention. The pilot will enroll 30 18-25 year olds who are identified through their case manager. Half of the participants will be randomized to receive the intervention and half to services as usual. Phase 4 data analyses will examine rates of research and treatment attrition, mean number of research and treatment sessions completed, mean treatment satisfaction ratings, and estimate the effect size of the intervention.