Project Summary/Abstract More than three-fifths of Americans with mental health disorders (MHD) have not received treatment in the prior year. Societally, under-utilization of mental health care has a substantial impact on disability and contributes to premature mortality through causes such as suicide. The problem of low mental health care use persists despite progress in increasing the rate of health insurance coverage. A major barrier to mental health services is cost, even among the majority of Americans who have either employer-sponsored or individual market commercial health insurance. As such policy interventions to reduce costs of mental health services among people with commercial insurance might have substantial effects on mental health services use and potentially quality. In 2021, New Mexico passed SB 317 “No Behavioral Health Cost Sharing,” which prohibited cost-sharing for mental health services in the commercial insurance plans that the state regulates. This law was the first of its kind nationally, and represents a stark natural experiment in reducing cost-related barriers to mental health services. The proposed study will assess the implementation and effects of the SB 317 No Behavioral Health Cost Sharing law with four specific aims. 1) Characterize the implementation of SB 317 from the perspective of clinicians and patients in New Mexico who are directly affected by the law, using qualitative interviews with key stakeholders. 2) Assess the effect of SB317 on MHD services benefit generosity using insurance documents that summarize plan benefits. 3) Evaluate the effect of SB317 on use of MHD services using quasi-experimental methods and comprehensive commercial claims data. 4) Evaluate the effect of SB317 on the quality of MHD services received. Understanding the way that zero cost-sharing affects mental health services will provide important new evidence to help guide mental health financing decisions to most- efficiently improve mental health services use.