The Paul Wellstone and Pete Domenici Mental Health Parity and Addiction Equity Act of 2008 (MHPAEA) requires that employers who offer health insurance coverage for mental health conditions and substance use disorders (MH/SU) provide coverage that is no more restrictive than that offered for medical and surgical conditions. Employers were required to comply with the law for coverage that began on or after October 3, 2009. The Department of Labor (DOL), the Department of Health and Human Services (HHS), and the Department of the Treasury share oversight for MHPAEA. MHPAEA also requires GAO to examine trends in health insurance coverage of MH/SU. This report describes (1) the extent to which employers cover MH/SU through private health insurance plans, and how this coverage has changed since 2008; and (2) what is known about the effect of health insurance coverage for MH/SU on enrollees’ health care expenditures; access to, or use of, MH/SU services; and health status. GAO surveyed a random sample of employers about their MH/SU coverage for the most current plan year and for 2008. GAO received usable responses from 168 employers–a 24 percent response rate. The survey results are not generalizable; rather, they provide information limited to responding employers’ MH/SU coverage. GAO reviewed published national employer surveys on health insurance coverage and interviewed officials from DOL, HHS, and other experts. GAO also reviewed studies that evaluated the effect of MH/SU coverage on enrollees’ expenditures, access to, or use of, MH/SU services, and health status. Most employers continued to offer coverage of MH/SU since MHPAEA was passed. Of the employers that responded to GAO’s survey, 96 percent offered coverage of MH/SU for the current plan year and for 2008, before MHPAEA was passed. Approximately 2 percent of employers reported offering coverage for only mental health conditions but not substance use disorders for the current plan year and for 2008. Conversely, about 2 percent of employers reported discontinuing their coverage of both MH/SU or only substance use disorders in the current plan year. The types of MH/SU diagnoses included and excluded in employers’ MH/SU benefits remained consistent between the current plan year and 2008. Of the employers who provided information about diagnoses included in their MH/SU benefits for both the current plan year and 2008, 34 percent reported that their most popular plan in the current plan year excluded at least one MH/SU diagnosis from their benefits, and 39 percent of employers reported excluding at least one MH/SU diagnosis from their benefits for the 2008 plan year. The most common change to MH/SU benefits reported among those who responded to the survey was enhancing benefits through the removal of treatment limitations, such as the number of allowed office visits. Reported use of lifetime dollar limits on MH/SU treatments also declined from 2008 to the current plan year. Among employers who reported information on cost-sharing, copayments and coinsurance amounts for in-network providers generally stayed about the same, fluctuating minimally from 2008 to the current plan year. Published national employer surveys on health insurance coverage also reported results consistent with GAO’s survey data. Employers may continue to modify certain nonfinancial requirements–such as changes to the services they cover (the scope of services) and nonquantitative treatment limits–in their MH/SU benefits in response to agencies’ issuance of final implementing regulations for MHPAEA. Officials from DOL and HHS reported that the final regulations may provide additional detail on these nonfinancial requirements. Research suggests that coverage for MH/SU has a varied effect on enrollees. Research examining the effect of health insurance coverage for MH/SU on enrollee expenditures generally found that the implementation of parity requirements reduced enrollee expenditures. Studies that examined the effect of health insurance coverage for MH/SU on enrollee access to, and use of, MH/SU services had mixed results, with some studies indicating there was little to no effect and others indicating that there was some effect–such as finding that restricting coverage had a negative effect on use of services. Little research has explored the relationship between health insurance coverage and health status. Of the studies we reviewed, two examined the effect of health insurance coverage for MH/SU on enrollee health status and found different effects. GAO provided a draft of the report to DOL and HHS. Both agencies provided technical comments, which have been incorporated as appropriate.