An ACO will fail without precise patient population definitions. ACOs need to define populations for many reasons, including identifying their members and attributing those patients to the correct physician and performing population health analytics. The challenges to a good population definition are: multiple providers per member, multiple data sources, and multiple identifiers for each member. Using a clinical integration hierarchy to refine population and subpopulations will solve a lot of these issues. A data warehouse is the foundation that makes it possible.