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.