PROJECT SUMMARY/ABSTRACT Substance use disorders (SUDs) can have devastating consequences for people with serious mental illness (SMI). SUDs can increase morbidity and mortality and are associated with higher healthcare and social costs, homelessness, and incarceration. Unfortunately, despite the availability of effective treatments, most individuals with co-occurring SMI and SUD, (COD) never receive SUD treatment--treatment that could significantly improve patient outcomes. Increasing access to SUD treatment and improving patient outcomes will require addressing both the supply of and the demand for treatment. At the system and provider level, the availability of treatment needs to be increased. At the patient level, patient demand for treatment needs to increase, by identifying and addressing patient perceptions of need for treatment and preferences. We propose to evaluate system, provider and patient-level facilitators and barriers, and then to use this information to develop an implementation strategy and toolkit to promote the use of medication assisted treatment (MAT)--the use of FDA-approved medications for SUD in combination with behavioral therapies--for people with COD receiving public mental health treatment. We focus on MAT because of its demonstrated effectiveness and cost-savings, and specialty mental health because of the relatively large role the mental health system plays in treating individuals with COD. Aims 1-3 seek to assess organizational capacity (at the system and provider level); organizational readiness (at the provider level); and perceived needs, attitudes and preferences (at the patient level) to identify barriers and facilitators. In Aim 4, we will use findings from Aims 1- 3 to guide development of the implementation strategy and toolkit, using stakeholder input and a systematic process for strategy development. By implementation strategy we mean a group of implementation interventions that will address barriers at multiple levels. By toolkit, we mean the resources providers and clinics will need to execute implementation. To conduct the research, we will collaborate with the Los Angeles County Department of Mental Health. Using a mixed methods approach, we will conduct semi-structured interviews, focus groups, and surveys with patients and providers from 8 clinics serving an ethnically and geographically diverse population. We plan to test the strategy in a subsequent R01, where we will examine how the strategy affects MAT adoption, implementation and sustainability. Our study is a first step towards increasing access to SUD treatment for a vulnerable, costly and underserved population. Our approach is innovative because we consider barriers to MAT from the system, provider, and patient perspectives simultaneously, and use this information to develop an implementation strategy that addresses both organizational supply and patient demand.