Receiver operating characteristic (ROC) analysis provides the most comprehensive description of diagnostic accuracy available to date, because it estimates and reports all of the combinations of sensitivity and specificity that a diagnostic test is able to provide. After sketching the 6 levels at which diagnostic efficacy can be assessed, this paper explains the conceptual foundations of conventional ROC analysis, describes a variety of indices that can be used to summarize ROC curves, and describes several forms of generalized ROC analysis that address situations in which more than 2 decision alternatives are available. Key issues that arise in ROC curve fitting and statistical testing are addressed in an intuitive way to provide a basis for judging the validity of ROC studies reported in the literature. A list of sources for free ROC software is provided. Receiver operating characteristic methodology has reached a level of maturity at which it can be recommended broadly as the approach of choice for radiologic imaging system comparisons.