Center effects are differences in outcome among treatment centers that cannot be explained by identifiable differences in patients treated or specific treatments applied and are presumed to result from differences in the ways health care is delivered. This paper will briefly review studies of association between treatment center factors and clinical outcomes in general medicine and surgery and look more closely at studies involving hematopoietic stem cell transplantation. We will also attempt to identify conceptual domains to study further the processes and mechanisms that may be associated with better outcomes.