Minimum nurse staffing levels have been promoted by researchers and legislators to reduce adverse events and improve patient safety in both hospitals and nursing homes. While this review of reviews found higher nurse staffing was generally associated with positive outcomes, results varied between staffing groups (e.g., registered nurses compared to licensed vocational/practical nurses or nursing assistants). The authors identified several methodological challenges and described how study design modifications could yield a more robust examination of the causal relationship between staffing and outcomes.