A shortage of registered nurses, in combination with increased workload, has the potential to threaten quality of care. Increasing the nurse to patient ratios has been recommended as a means to improve patient safety. However, the cost effectiveness of increasing registered nurse (RN) staffing is controversial. This systematic review analyzes associations between hospital nurse staffing and patient outcomes with consideration of variables that could influence the primary association. The basic research questions were: 1. How is a specific nurse to patient ratio associated with patient outcomes (i.e., mortality; adverse drug events, nurse quality outcomes, length of stay; patient satisfaction with nurse care)? How does this association vary by patient characteristics, nurse characteristics, organizational characteristics, and nursing outcomes? 2. How is a measure of nurse work hours (hours per patient or patient day) associated with the same patient outcomes? 3. What factors influence nurse staffing policies? 4. What nurse staffing strategies are effective for improving the patient outcomes listed in question 1? 5. What gaps in research on nurse staffing and patient outcomes can be identified to address in future studies? Questions 1, 2, and 4 are addressed in the systematic review using meta-analytic approaches. The literature associated with question 3 does not lend itself to meta-analysis. Questions 1 and 2 address the same basic association but employ two different measures of nurse staffing. The nurse to patient ratio relies on a general ratio, which may include all nurses assigned to a unit, including non-clinical time, whereas nurse work hours look specifically at nurses involved in patient care. Even beyond this distinction, the varied ways staffing rates are calculated complicates pooling data.