The World Health Organization surgical safety checklist garnered a great deal of attention after initial studies showed remarkable reductions in postoperative complication rates. However, subsequent studies failed to reproduce these results, engendering controversy about the true effectiveness of checklists in real-world settings. This systematic review of 25 studies of the surgical safety checklist found that complication rates decreased with checklist usage in resource-poor settings, but the checklist did not appear to be effective in developed nations. The authors also noted that the reported effect of the checklist was incongruous—in several studies, postoperative complications did not decrease, but postoperative mortality improved, raising questions about what mechanism helped the checklist achieve its effect. These concerns, along with methodological problems in many of the included studies, led the authors to postulate that the observed improvements seen in some studies may have been due to temporal changes or other interventions rather than the checklist itself.