Failure to address both operational and cultural factors in cardiac surgery has led to serious safety problems and preventable deaths, most notably at the Bristol Royal Infirmary. This study used detailed observation of cardiac surgical procedures by a multidisciplinary team, including clinicians and human factors engineering specialists, to prospectively identify safety hazards. Many types of hazards were identified, including problems with communication and teamwork, poor interoperability of equipment, and failure to follow established safety protocols. The authors make detailed recommendations to guide institutions in addressing these problems.