The last two decades have seen dramatic growth in the share of Medicare beneficiaries enrolled in private managed care plans, now known as Medicare Advantage (MA). Since beneficiaries have multiple opportunities to change their health care plan during enrollment, this growth may stem from increases in the share of new Medicare enrollees choosing MA and enrollees in fee-for-service, or traditional Medicare (TM), deciding to switch to an MA plan. Because MA enrollees are also permitted to switch to TM, a reduction in switching from MA to TM may also contribute to overall MA growth. In this brief, we examine recent trends in behaviors contributing to MA growth and describe the beneficiary and program factors associated with each decision type. We found that the bulk of MA growth appears to be driven by increases in the fraction of new enrollees choosing MA plans. We found that switches during the annual open enrollment period were still rare, but there was a clear pattern of increasing likelihood of enrollees dropping TM in favor of an MA plan, especially in the most recent decade. Finally, we found that switching probabilities vary greatly by state and that switching from TM to MA was less likely among beneficiaries with chronic diseases.