The Affordable Care Act (ACA) has prompted health plans to increase their use of “narrow networks” of providers as a cost containment strategy. The Leonard Davis Institute of Health Economics (LDI) has assembled the first integrated dataset of physician networks for the plans offered on the ACA marketplace. This data brief uses this new resource to describe the breadth of the physician networks in plans sold on the state and federal marketplaces. The percent of physician networks that were classified as small or x-small came to 41% overall, 55% for HMO networks, and 25% for PPO networks.