Legislation in the United States regulating nurse staffing has increased on a state level and has been discussed at the Federal level. This increase has been justified by research that shows a negative association between inadequate nurse staffing and patient outcomes. Using a systematic review of the literature, the association between nurse staffing and adverse patient outcomes, including mortality and failure to rescue, is assessed. Literature supports that factors associated with nurse staffing—skill mix, education, work environment, and patient ratios—do have an impact on patient mortality. The effect of these staffing factors is not yet well understood and more research is needed to determine causal interactions.