Objective To examine the extent to which accountable care organizations (ACOs) formally incorporate postacute care providers. Data Sources The National Survey of ACOs (N = 269, response rate 66 percent). Study Design We report statistics on ACOs' formal inclusion of postacute care providers and the organizational characteristics and clinical capabilities of ACOs that have postacute care. Principal Findings Half of ACOs formally include at least one postacute service, with inclusion at higher rates in ACOs with commercial (64 percent) and Medicaid contracts (70 percent) compared to ACOs with Medicare contracts only (45 percent). ACOs that have a formal relationship with a postacute provider are more likely to have advanced transition management, end of life planning, readmission prevention, and care management capabilities. Conclusions Many ACOs have not formally engaged postacute care, which may leave room to improve service integration and care management.