In science and medicine, images are acquired for specific purposes or tasks, and the ultimate measure of image quality is how well the task is performed. In mammography, for example, the task is detection of breast tumors, and in several forms of cardiac imaging the task is estimation of the cardiac ejection fraction. The term “observer” refers to the method by which the task is performed; for tumor detection the observer can be either a radiologist or a computer algorithm, and for estimation of ejection fraction it is usually an image analysis program. In either case, the average performance of the observer is a measure of the quality of the imaging system for that particular task and observer, and for the particular class of subjects being imaged. This performance is influenced by many factors, including the object itself, any contrast agent used, the nature of the image-acquisition system, subsequent image reconstruction, processing and display; and the capabilities of the observer, be it human or machine.