This report takes an in-depth look at Medicare Advantage plans’ hospital networks. The analysis draws upon data from 409 Medicare Advantage plans serving beneficiaries in 20 diverse counties that together accounted for about one in seven (14%) Medicare Advantage enrollees nationwide in 2015. The report examines the size and composition of plans’ hospital networks, the variation across counties, the inclusion of Academic Medical Centers and NCI-Designated Cancer Centers, and the relationship between network size and other plan features, including premiums, quality star ratings, per capita Medicare spending, parent organization, and plan tax status.