Project Summary Adolescent substance use disorder is a public health crisis with over 1 million youth and young adults affected each year in the United States. County-level policies are well-positioned to reduce adolescent substance use disorder through funding and regulatory policies; however, the extant literature documents multiple failures in attempts to effectively leverage policy mechanisms. The challenges of aligning political will, multisector collaboration, use of research evidence, and resource commitments are well-known problems in the public policy and public administration literature. To date, the field of behavioral health policy science does not have a well-supported method of policy design that overcomes downstream policy implementation challenges. We propose to study a method of policy formation, Policy Codesign, designed to anticipate multiple barriers in the downstream implementation of evidence-based, adolescent SUD services. The participant structure and process of Policy Codesign draws from the public policy model of coproduction as well as engineering-based codesign and is supported by promising, preliminary studies. The process includes well-defined policy formation stages within a multisector and community-engagement framework, including: Values mapping, Information gathering, Information integration, Prototyping, Testing, Implementation. The current single-arm, observational study with two geographically diverse counties in Washington State will inform the research procedures to prepare for a rigorous trial of Policy Codesign in counties in Washington State, New York, and Connecticut. The project’s specific aims are 1) to examine the acceptability of Policy Codesign and perceived feasibility of developed adolescent SUD policies, 2) to measure changes in social network growth, cohesion, and bridging activities among multiple sectors central to behavioral health policy implementation, and 3) to examine the perceived replicability of Policy Codesign among well-established behavioral health policy intermediaries. The project is innovative as the first test of a policy design strategy intended to solve downstream implementation problems in evidence-based, adolescent SUD prevention. If found to be effective, Policy Codesign would significantly advance the field’s ability to translate and sustain policy-based solutions to address adolescent substance use. The core investigative team (Walker, Ahrens, Owens) is well-suited to carry out the project with extensive practical expertise in real world behavioral health policy design and community-engaged research. The team is supported by leading policy (Purtle), implementation (Saldana, Palinkas, Aarons) and adolescent SUD treatment (Hogue) researchers widely regarded as the top experts in their fields, as well as behavioral health policy and health design organizations actively involved with county policymaking in adolescent behavioral health systems.