The Affordable Care Act (ACA) introduced several changes to Medicare Advantage (MA) plan payment to better align Medicare spending on MA enrollees with average spending per traditional Medicare enrollee. These payment changes were expected to reduce enrollment in MA plans by making private plans less attractive to both insurers and beneficiaries, but MA enrollment has nearly doubled since ACA implementation. This report disaggregates MA enrollment growth by plan type and geography to better understand variation in the growth of Medicare private plans and explores county-level market and demographic changes associated with increased MA penetration.