Receiver operating characteristic (ROC) analysis measures the "diagnostic accuracy" of a medical imaging system, which represents the second level of diagnostic efficacy in the hierarchical model described by Fryback and Thornbury (Med Decis Making 11:88-94, 1991). After describing the historical origins of ROC analysis, this paper reviews the importance of sampling cases appropriately, designing an observer study to avoid bias, and collecting data on a useful scale. A variety of methods for fitting ROC curves to observer data and testing the statistical significance of apparent differences are then reported. Finally, generalized forms of ROC analysis that require lesion localization or allow more than two states of truth are surveyed briefly.