On January 1, 2010, the Paul Wellstone and Pete Domenici Mental Health Parity and Addiction Equity Act takes effect for most insurance plans. As the December issue goes to press, federal policy makers have not yet issued implementation guidelines. These officials and thousands of individuals in private industry and state monitoring agencies who will be charged with ensuring adherence to the guidelines might learn from California, where parity has been a reality since 2000. This month's lead article reports experiences of California health plans, providers, and consumers during the first five years of parity. From September 2001 through January 2006, Margo L. Rosenbach, Ph.D., and colleagues conducted interviews with nearly 150 state- and community-based stakeholders and led six focus groups that included 84 providers and consumers. The researchers found that although health plans eliminated differential benefits and cost-sharing requirements for the limited list of diagnoses to which the law applies, their use of medical necessity criteria to control costs led to concerns about access and quality, which in turn led to an increase in regulatory oversight five years after implementation. Health plan executives concluded that use of the limited list of diagnoses, which had some unintended consequences, was unnecessary. Lack of consumer knowledge about parity was widely acknowledged to be a critical problem (page 1589).