High-sensitivity cardiac troponin (hs-cTn) assays have been used clinically by thousands of physicians in many countries throughout the world since their clinical introduction 7 years ago. In the early diagnosis of myocardial infarction (MI), beyond doubt, the most important indication of hs-cTn assays, these simple, inexpensive, and highly reproducible tools complement detailed clinical assessment including chest pain characteristics and the electrocardiogram. Hs-cTn assays for the first time allowed the precise quantification of cardiomyocyte injury around the 99th percentile and thereby substantially increased the accuracy of MI detection from blood obtained at presentation to the emergency department (ED). Higher accuracy at ED presentation enabled the development and extensive validation of early hs-cTn–based diagnostic algorithms, which substantially reduced the time required for the safe rule-out or rule-in of MI. This review summarizes key principles underlying the safe and effective use of hs-cTn in the ED in patients with suspected MI. Display omitted