Born as an afterthought to Medicare five decades ago, Medicaid has evolved from an adjunct to state welfare programs to the nation’s largest health insurer. The occasion of Medicaid’s 50th birthday is a fitting time to consider that evolution, not to reminisce, but to help chart the path forward. Medicaid is a complex program with a complex history, and understanding its role in the U.S. health system is essential to ensuring that it is performing at optimal levels for its beneficiaries, as well as for states, taxpayers, and the myriad healthcare providers, health plans, and others touched by the program. In a four-part series supported by The Commonwealth Fund, Cindy Mann, Deborah Bachrach and Elizabeth Osius of Manatt Health examine Medicaid’s evolution and consider its role in the new coverage paradigm established by the Affordable Care Act (ACA).