The remarkable initial success of the World Health Organization surgical safety checklist led to the United Kingdom's National Health Service mandating its use in 2009. Subsequent studies of the checklist, however, have failed to demonstrate improvements in perioperative complication rates. This study analyzed the relationship between checklist implementation and complication rates at five hospitals in the UK. The investigators found that the checklist was effective only when it was fully completed—the odds of a postoperative complication were reduced by more than 40% if the full checklist was completed, but this was done in only 62% of cases. Moreover, even complete checklist usage did not seem to prevent complications in low-risk patients. The results of this and other studies clearly demonstrate that a checklist is a complex intervention that requires rigorous implementation and monitoring in order to improve safety.