The Center for Medicare & Medicaid Innovation (CMMI) in the Centers for Medicare & Medicaid Services (CMS) has selected Mathematica Policy Research to evaluate the largest and most ambitious reform of primary care payment and delivery ever tested in this country—the Comprehensive Primary Care Plus (CPC+) initiative. Primary care is an essential component of effective health care delivery, and Mathematica researchers will apply their decades of experience studying primary care as they test and rigorously evaluate the transformative effects of CPC+ in 14 regions across the United States.