Although checklists have been shown to improve safety and surgical mortality, they can be difficult to implement, which limits their effectiveness in clinical practice. This study examined whether perceptions of teamwork predicted checklist performance. Trained observers used standardized tools to rate the extent of checklist completion and quality of teamwork. They found that checklists were implemented as intended in only 3% of cases. Surgical teams with better surgeon buy-in to checklists, clinical leadership, communication, and overall teamwork completed more checklist components. Clinical factors, including older patient age and longer duration of surgery, were also associated with performing more of the checklist. The authors suggest that teamwork is critical to checklist implementation. A PSNet interview discussed the challenges of implementing checklists in health care.