Take a quick look at the competitive makeup of health care organizations in any single market in the United States, and you are likely to find at least one, if not several, hospital-based health care systems that serve as the primary and dominant health care providers within their local communities. Indeed, the existence of these entities, most of which are clusters of two or more acute care hospitals and other associated health care businesses, and the prominent role they play in the US health care system may seem obvious to the casual observer. Ironically, despite their importance as delivery modalities and their potential effects on competition and policy, these distinctive organizational forms—the clusters—are strikingly understudied. The gaps in knowledge appear even at the most basic levels of analysis, including their measurement and conceptualization. What are these systems? How did they come to exist? And how does their emergence and development speak to our understanding of organizations, organizational forms, and organization theory?