The Affordable Care Act hastened the emergence of a new healthcare delivery and payment model called Accountable Care Organizations (ACOs). ACOs are groups of healthcare providers linked through a common governance structure that are responsible for serving “aligned” patients who seek most of their primary care from those providers. ACOs also include a shared savings opportunity – if providers can manage costs associated with aligned patients underneath a pre-determined benchmark, while meeting quality performance expectations, they can access savings. Based upon provisions in the Affordable Care Act, CMS initiated two Medicare ACO programs in 2012, discussed in detail in this chapter: the Medicare Shared Savings Program (MSSP) and the Pioneer ACO Program. While novel programs, the ACO concept draws upon a lengthy political and health policy history, also reviewed herein.