Over the past 25 years, health care organization scholars have observed the dramatic emergence of hospital-based clusters in local markets throughout the U.S. These important organizational forms require same-system ownership of multiple general, acute care hospitals operating within a single local market, and as such they include multi-hospital systems that are entirely contained in a single urban market as well as clustered extensions or subsystems of larger regional and national systems. However, despite their noted growth as powerful forces in local markets, relatively few studies have examined these clusters, and as a result there remains a significant gap in our knowledge regarding their continued growth or the diverse components and configurations they may exhibit. This study endeavors to both describe and explain the diversity observed across hospital-based clusters. To fulfill this objective, a national inventory of clusters is updated to reflect cluster membership as of 2012, and a catalog of cluster components – including their hospital-based and non-hospital-based sites – is created, acknowledging that clusters today consist of more than just general, acute care hospitals. Cluster analysis methods are then employed to develop a taxonomy of cluster forms, using a sample of 114 clusters from local markets in Florida, Maryland, Nevada, Texas, Virginia, and Washington. Applying a conceptual framework informed by concepts from contingency theory and strategic management theory, cluster analysis methods yield a five-group solution, which is then externally validated using a multi-theoretical perspective synthesizing arguments from population ecology, institutional theory, industrial organization economics, transaction cost economics, and resource dependence theory. Results from descriptive and multinomial logistic regression analyses identify organizational and environmental factors that are significantly associated with various cluster forms. The study’s results suggest that today’s hospital-based clusters continue to grow and vary according to the dimensions of differentiation-configuration and integration-coordination. These findings provide a foundation for future examinations of hospital-based clusters, including their provision of services within and outside of hospital walls. These results also accentuate the importance of accounting for geographic considerations when examining health care organization forms, and they display the utility and value of employing a multi-theoretical perspective to examine and explain such complex forms.