Objective: To understand the organization of cardiologists in the United States and territories with an emphasis on identifying the characteristics of practices where cardiologists practice. Data: The Healthcare Organization Services database, which provides information on the designation, specialties and business associations, or affiliations, of healthcare professionals across the US and territories. These data were updated as of September 2013. Methods: Cardiologists were defined as providers with an MD or DO, who have an active clinical practice, and either: (1) have Cardiology as primary specialty, or (2) who have Internal Medicine, Pediatrics or Family Medicine indicated as primary specialty, with Cardiology as secondary specialty. Cardiologists were linked to businesses and descriptive analyses were conducted to characterize the organization of cardiologists. Findings: We identified 33,745 cardiologists; 22,461 of these cardiologists had at least one affiliation with an ambulatory practice or independent physician association (AP/IPA), while 30,567 had at least one hospital affiliation. Approximately 57.6% (19,453 of 33,745) of cardiologists were affiliated with both the AP/IPA and hospital settings. The overwhelming majority of ambulatory practices linked to hospital-AP/IPA affiliated cardiologists are medical groups (7,839 of 9,838), while independent physician practices comprise the next largest category (1,498 of 9,838). Of the 7,839 medical groups, 3,809 are cardiology specialty, 1,316 are multispecialty group practices, and 2,060 are family practice, general practice, geriatric medicine, internal medicine, pediatric medicine or primary care. Among all non-multispecialty medical groups (n=6,523), 59.2% (3,862 of 6,523) have 1-2 affiliated cardiologists, 21.7% (1,413 of 6,523) have 3-5, 12.6% (821 of 6,523) have 6-10 and 6.5% (427 of 6,523) have more than 10 affiliated cardiologists. Focusing at the provider level, 17,639 hospital-AP/IPA cardiologists had any affiliation with a medical group (MG), with 12,840 cardiologists with any cardiology specialty MG affiliation, 3,215 with any multispecialty group practice MG affiliation, and 3,051 with any family practice, general practice, geriatric medicine, internal medicine, pediatric medicine or primary care MG affiliation. Overall, 15,476 hospital-AP/IPA cardiologists had any affiliation with at least one non-multi-specialty group practice (non-MSGP), with 4,279 affiliated with one or more non-MSGP(s) with 1-2 cardiologists, 4,378 with one or more non-MSGP(s) with 3-5 cardiologists, and 4,861 and 5,099 affiliated with one or more non-MSGP(s) with 6-10 and >10 cardiologists, respectively. Conclusion: The majority of cardiologists were affiliated with both a hospital and ambulatory practice or IPA. Of this subset, most cardiologists were affiliated with at least one group practice.