Medicare Advantage enrollment has almost doubled since 2010 and now accounts for more than a third of all Medicare beneficiaries. We performed a systematic review to compare Medicare Advantage and traditional Medicare on key metrics. Evidence from forty-eight studies showed that in most or all comparisons, Medicare Advantage was associated with more preventive care visits, fewer hospital admissions and emergency department visits, shorter hospital and skilled nursing facility lengths-of-stay, and lower health care spending. Medicare Advantage outperformed traditional Medicare in most studies comparing quality-of-care metrics. However, the evidence on patient experience, readmission rates, mortality, and racial/ethnic disparities did not show a trend of better performance in Medicare Advantage. Evidence to date might not fully account for selection bias, unobserved differences in social determinants of health, or risk adjustment challenges, in part because of differences in data quality that limit the comparability of outcomes between Medicare Advantage and traditional Medicare. With Medicare Advantage plans expected to grow in popularity, policy makers should support policies to improve data completeness and comparability, and health plans should focus on improving patient experience.