There have been numerous significant clinical advances in both the diagnosis and therapy of acute coronary syndrome during the past several years. Even the term "acute coronary syndrome" is a recent creation meant to expand clinical attention in patients with chest pain of coronary origin beyond identification of ST-segment elevation myocardial infarction and prompt initiation of reperfusion therapy and to include the evaluation and management of those patients with unstable angina (UA) or myocardial injury that does not cause ST-segment elevation. Many of these advances have been studied and first implemented outside the emergency department, leading some emergency physicians to be slow to embrace them, and leaving others without a viable practical option to use them outside of the cardiac catheterization laboratory or the coronary care unit. In September 2000, the American College of Cardiology and the American Heart Association issued practice guidelines for the care of patients with UA and non-ST-segment elevation myocardial infarction. The guidelines specifically address the diagnosis and management of UA and non-ST-segment elevation myocardial infarction in the ED, suggesting evidence-based standards for risk stratification, for the use of biologic markers of myocardial damage and other adjunctive diagnostic tests, and for the appropriate use of antiplatelet and antithrombin therapeutic agents. This article provides an overview of the ED-pertinent analyses and recommendations from the 93-page document. A commentary on the implementation of these recommendations in the ED follows in a separate article.