Over the past twenty-five years, thirty-seven states and the US Congress have passed mental health and substance use disorder (MH/SUD) parity laws to secure nondiscriminatory insurance coverage for MH/SUD services in the private health insurance market and through certain public insurance programs. However, in the intervening years, litigation has been brought by numerous parties alleging violations of insurance parity. We examine the critical issues underlying these legal challenges as a framework for understanding the areas in which parity enforcement is lacking, as well as ongoing areas of ambiguity in the interpretation of these laws. We identified all private litigation involving federal and state parity laws and extracted themes from a final sample of thirty-seven lawsuits. The primary substantive topics at issue include the scope of services guaranteed by parity laws, coverage of certain habilitative therapies such as applied behavioral analysis for autism spectrum disorders, credentialing standards for MH/SUD providers, determinations regarding the medical necessity of MH/SUD services, and the application of nonquantitative treatment limitations under the 2008 federal parity law. Ongoing efforts to achieve nondiscriminatory insurance coverage for MH/SUDs should attend to the major issues subject to private legal action as important areas for facilitating and monitoring insurer compliance.