INTRODUCTION: Poor physician performance has a profound impact on patient safety and society's trust in the health care system. The attention that this topic has received in the media suggests that it is a large-scale issue. However, research about physician performance is still scant; there is little evidence regarding its prevalence. In terminology, characteristics and causes of poor performance seem to be used synonymously. The aim of this study was to describe (i) characteristics of poor performance, (ii) causes contributing to its onset and continuation, and (iii) prevalence of poor performance among physicians in the Netherlands. METHODS: This is a mixed-methods study involving literature review of 10 electronic databases, review of disciplinary law verdicts, and 12 expert interviews. Each of the 3 methods contributes to the aims of our study. RESULTS: Characteristics of poor performance are predominantly described by deficits in individual physician knowledge, skills, and behavior. Causes of poor performance include aspects such as poor collaboration, lack of criticism, insufficient leadership, and lack of professional development. A prevalence rate of 5% was judged by the experts to be realistic; evidence to underpin this number is lacking. CONCLUSIONS: This study discriminates between characteristics and causes of poor performance. Characteristics of poor performance are related to individual physician aspects. Causes contributing to the onset and continuation of poor performance include not only individual components but also work environment and professional development. Our findings therefore underscore the importance of considering poor performance on a system level rather than as a pure individual physician issue.