Do the working conditions of health care personnel contribute to the incidence of medical errors? This question is often raised during public discussion of ways to improve patient safety. How much do issues of nurse staffing and doctors' hours, for example, contribute to the estimated 44,000 to 98,000 deaths per year in hospitals due to medical errors? The objective of this report is to identify and summarize evidence from the scientific literature on the effects of health care working conditions on patient safety. The report also identifies relevant information from industries outside of heath care. Working conditions were classified into five categories: workforce staffing, workflow design, personal/social factors, physical environment, and organizational factors. The classification system for working conditions was derived from existing literature and advice from an expert panel. It is consistent with human factors research in multiple disciplines and industries such as aviation and nuclear power. Workforce staffing refers to job assignments and includes four principal aspects of job duties: the volume of work assigned to individuals, the professional skills required for particular job assignments, the duration of experience in a particular job category, and work schedules. Workflow design focuses on the job activities of health care workers, including interactions among workers and the nature and scope of the work as tasks are completed. Personal/social factors refer to individual and group factors such as stress, job satisfaction, and professionalism. Physical environment includes aspects of the health care workplace such as light, aesthetics, and sound. Organizational factors are structural and process aspects of the organization as a whole, such as use of teams, division of labor, and shared beliefs. The researchers developed an analytic framework to define how working conditions are related to patient safety. Antecedent conditions, which are external factors such as personal characteristics of workers and fixed structural characteristics of the system (e.g., geographic location, regulations, and legislation), can affect the impact of working conditions on patient safety. Working conditions are viewed either as resources that improve work quality or as demands that impede work quality. Working conditions potentially affect patient safety, which leads to patient outcomes. The researchers also developed a model of patient safety to help frame the key questions and provide a way to synthesize data reported in studies. The model is drawn from injury analysis and incorporates elements of both processes and outcomes. It is based on the relationships between medical errors (defined as the failure of a planned action to be completed as intended, or the use of a wrong plan) and adverse outcomes (injuries caused by health care rather than underlying disease).