A challenge to investigating quality of care at safety-net hospitals is the absence of a standard method for identifying these hospitals. The authors identified three different, commonly used approaches for classifying hospitals as safety-net providers. Analyzing national data on hospital demographics and quality of care, they found little overlap among these three sets of hospitals. Under two definitions, safety-net providers clearly underperformed on quality compared with non-safety-net providers; under a third definition, results were mixed. How one defines safety-net providers can affect health services research outcomes and policy recommendations.