Rationale: Effective teamwork is fundamental to the management of medical emergencies, and yet the best method to teach teamwork skills to trainees remains unknown.Objectives: In a cohort of incoming internal medicine interns, we tested the hypothesis that expert demonstration of teamwork principles and participation in high-fidelity simulation would each result in objectively assessed teamwork behavior superior to traditional didactics.Methods: This was a randomized, controlled, parallel-group trial comparing three teamwork teaching modalities for incoming internal medicine interns. Participants in a single-day orientation at the Vanderbilt University Center for Experiential Learning and Assessment were randomized 1:1:1 to didactic, demonstration-based, or simulation-based instruction and then evaluated in their management of a simulated crisis by five independent, blinded observers using the Teamwork Behavioral Rater score. Clinical performance was assessed using the American Heart Association Advanced Cardiac Life Support algorithm and a novel "Recognize, Respond, Reassess" score.Measurements and Main Results: Participants randomized to didactics (n = 18), demonstration (n = 17), and simulation (n = 17) were similar at baseline. The primary outcome of average overall Teamwork Behavioral Rater score for those who received demonstration-based training was similar to simulation participation (4.40 6 1.15 vs. 4.10 6 0.95, P = 0.917) and significantly higher than didactic instruction (4.40 6 1.15 vs. 3.10 6 0.51, P = 0.045). Clinical performance scores were similar between the three groups and correlated only weakly with teamwork behavior (coefficient of determination [R s 2 ] = 0.267, P , 0.001).Conclusions: Among incoming internal medicine interns, teamwork training by expert demonstration resulted in similar teamwork behavior to participation in high-fidelity simulation and was more effective than traditional didactics. Clinical performance was largely independent of teamwork behavior and did not differ between training modalities.