The first decade of the patient safety movement achieved some real gains, focused as it was on adverse events amenable to systemwide solutions, such as infections associated with health care and medication errors. However, diagnostic errors, although common and often serious, have not received comparable attention. They are challenging to measure and less amenable to systemwide solutions. Furthermore, it is difficult to hold hospitals accountable, since diagnostic errors usually result from cognitive mistakes on the part of one or more members of the medical staff. Health information technology, better training, and increasing acknowledgment of the problem hold some promise. As approaches to measuring, preventing, and mitigating harm from diagnostic errors are proven to work, it will be important to integrate these approaches into policy initiatives to improve patient safety.