Over the past 25 years, Indian gaming has become a significant source of revenue for many tribes, reaching $28 billion in fiscal year 2013. IGRA, the primary federal statute governing Indian gaming, provides a statutory basis for the regulation of Indian gaming. Tribes, states, Interior, and the Commission have varying roles in Indian gaming. GAO was asked to review Indian gaming. This report examines (1) Interior's review process to help ensure that tribal-state compacts comply with IGRA; (2) how states and selected tribes regulate Indian gaming; (3) the Commission's authority to regulate Indian gaming; and (4) the Commission's efforts to ensure tribes' compliance with IGRA and Commission regulations. GAO analyzed compacts and Commission data on training, compliance, and enforcement; and interviewed officials from Interior, the Commission, states with Indian gaming, and 12 tribes in six states GAO visited selected for geographic distribution and gaming revenues generated.