The clinician looking, listening, or feeling for signs of a disease may far prefer a false alarm to a miss, particularly if the disease is serious and contagious. On the other hand, he may believe that the available therapy is marginally effective, expensive, and debilitating. The pilot seeing the landing lights only when they are a few yards away may decide that his plane is adequately aligned with the runway if he is alone and familiar with that plight. He may be more inclined to circle the field before another try at landing if he has many passengers and recent memory of another plane crashing under those circumstances. The Food and Drug administrator suspecting botulism in a canned food may not want to accept even a remote threat to the public health. But he may be less clearly biased if a recent false alarm has cost a canning company millions of dollars and left some damaged reputations. The making of almost any fine discrimination is beset with such considerations of probability and utility, which are extraneous and potentially confounding when one is attempting to measure the acuity of discrimination per se. The ROC is an analytical technique, with origins in statistical decision theory and electronic detection theory, that quite effectively isolates the effects of the observer's response bias, or decision criterion, in the study of discrimination behavior. This capability, pursued through a century of psychological testing, provides a relatively pure measure of the discriminability of different stimuli and of the capacity of organisms to discriminate. The ROC also treats quantitatively the response, or decision, aspects of choice behavior. The decision parameter can then be functionally related to the probabilities of the stimulus alternatives and to the utilities of the various stimulus-response pairs, or to the observer's expectations and motivations. In separating and quantifying discrimination and decision processes, the ROC promises a more reliable and valid solution to some practical problems and enhances our understanding of the perceptual and cognitive phenomena that depend directly on these fundamental processes. In several problem areas in psychology, effects that were supposed to reflect properties of the discrimination process have been shown by the ROC analysis to reflect instead properties of the decision process.