The Affordable Care Act (ACA) seeks to change fundamentally the US health care system. The responses of states have been diverse and changing. What explains these diverse and dynamic responses? We examine the decision making of states concerning the creation of Pre-existing Condition Insurance Plan programs and insurance marketplaces and the expansion of Medicaid in historical context. This frames our analysis and its implications for future health reform in broader perspective by identifying a number of characteristics of state-federal grants programs: (1) slow and uneven implementation; (2) wide variation across states; (3) accommodation by the federal government; (4) ideological conflict; (5) state response to incentives; (6) incomplete take-up rates of eligible individuals; and (7) programs as stepping-stones and wedges. Assessing the implementation of the three main components of the ACA, we find that partisanship exerts significant influence, yet less so in the case of Medicaid expansion. Moreover, factors specific to the insurance market also play an important role. Finally, we conclude by applying the themes to the ACA and offer an outlook for its continuing implementation. Specifically, we expect a gradual move toward universal state participation in the ACA, especially with respect to Medicaid expansion.