Accountable care organizations (ACOs) are emerging across the healthcare marketplace and now include Medicare, Medicaid, and private sector payers covering more than 24 million lives. However, little is known about the process of organizational change required to achieve cost savings and quality improvements from the ACO model. This study applies the complex innovation implementation framework to understand the challenges and facilitators associated with the ACO implementation process. We conducted four case studies of private sector ACOs, selected to achieve variation in terms of geography and organizational maturity. Across sites, we used semistructured interviews with 68 key informants to elicit information regarding ACO implementation. Our analysis found challenges and facilitators across all domains in the conceptual framework. Notably, our findings deviated from the framework in two ways. First, findings from the financial resource availability domain revealed both financial and nonfinancial (i.e., labor) resources that contributed to implementation effectiveness. Second, a new domain, patient engagement, emerged as an important factor in implementation effectiveness. We present these deviations in an adapted framework. As the ACO model proliferates, these findings can support implementation efforts, and they highlight the importance of focusing on patients throughout the process. Importantly, this study extends the complex innovation implementation framework to incorporate consumers into the implementation framework, making it more patient centered and aiding future efforts.