We searched MEDLINE (Ovid Technologies, 1966 to June 2004; English language) for terms describing physician experience (keywords: physician age, clinician age, physician experience, clinician experience), physician demographic characteristics (keywords: physician characteristics, clinician characteristics), practice variation (subject heading: physician's practice patterns), and performance in various domains (subject headings: clinical competence, health knowledge, attitudes and practice, outcomes assessment[health care]; keywords: knowledge, guideline adherence, appropriateness, outcomes). We retrieved potentially relevant articles and reviewed their reference lists to identify studies that our search strategy may have missed (Figure 1). We also searched our personal archives to identify additional studies. We included studies if they 1) were original reports providing empirical results; 2) measured knowledge, guideline adherence, mortality, or some other quality-of-care process or outcome; and 3) included years since graduation from medical school, years since certification, or physician age as a potential explanatory variable. We excluded studies if they described practice variation that is not known to affect quality of care (for example, assessed test-ordering behavior in clinical situations where optimal practice is unknown) or evaluated the performance of fewer than 20 physicians. For studies that examined several different end points, we included only those outcomes that are linked to knowledge or quality of care.