The Mental Health Parity and Addiction Equity Act of 2008 (MHPAEA) is a federal law that prevents group health plans from imposing less favorable limitations on mental health and substance use disorder benefits compared to medical and surgical coverage. The Affordable Care Act (ACA) builds on MHPAEA by requiring certain health plans to cover these services as essential health benefits, expanding protections effective from January 1, 2014, for individual and small group market plans. The final rules clarify compliance requirements and participant rights regarding mental health and substance use disorder benefits, including the need for transparency and adherence to parity regulations.