What are the factors in physicians’ decisions to treat Medicaid and uninsured patients and how have those factors and decisions have changed over time? Specifically, the researchers addressed four questions: 1) Have the determinants of physician participation in Medicaid changed since the mid-1980s? 2) Have the factors influencing physicians’ decisions to treat uninsured patients changed in recent years? 3) Which physicians have discretion about whether they accepted underserved patients and how does that affect the number of patients they treat? 4) To what extent are Medicaid patients and uninsured patients concentrated into the practices of a few physicians? As part of these analyses, the research team examined the proportion of physicians treating Medicaid and uninsured patients, including how that number has changed over time. They also looked at the effects of the changing health care market (i.e., growth of managed care and changes in physician organization) on physicians’ decisions. The objective of the project was to provide policymakers with more information about the factors that influence physicians’ decisions to treat underserved populations.