The Community Tracking Study (CTS) is a comprehensive, longitudinal investigation into health system changes across the United States, involving approximately 60,000 individuals and around 12,000 physicians from 60 nationally representative communities. Initially, 48 of these communities were large metropolitan areas, with 12 selected for in-depth study through larger survey samples and site visits. The study includes a nationally representative cohort comprised of households, health plan leaders, and physicians, each providing detailed information on various aspects of health care. Participants for the Household Survey were recruited through random sampling, while thousands of physicians contributed data through the Physician Survey and interviews. Additionally, an Employer Health Insurance Survey was conducted exclusively during the first round of data collection (1996-1997). Since its inception, the study has continued to collect data at two-year intervals. In the fifth round of data collection (2007), the community-based design was replaced by a national-sample design, although site visits continued to focus on the 12 communities. Subsequent rounds are referred to as the Health Tracking Household Survey. The study has completed six rounds of site visits, with the latest occurring in 2010-2011.