Diagnostic ultrasound has been useful to emergency physicians for more than three decades, generally as a formal imaging procedure performed by specialized physicians and technicians in a specialized unit. The excitement that surrounds emergency ultrasound today reflects a major change, perhaps a revolution, in the manner in which ultrasound technology is being applied in emergency departments (EDs) across the United States. In the past several years, it has become apparent that emergency physicians may, themselves, perform ultrasonography at the patient bedside for a variety of ED conditions. These brief and highly focused examinations are quite distinct from the traditional imaging studies performed by colleagues in other specialties (such as radiology, cardiology, and the vascular laboratory) and are best thought of as applications of a clinical tool, analogous to the slit lamp, proctoscope. As such, it has distinct advantages and limitations, both of which should be understood not only by the emergency physician but by the traditional imaging specialist and the patient's referring physician as well. The differences between these two quite different uses of ultrasound technology are so important for the successful use of emergency ultrasound (and such a common cause for the resistance to ultrasound in the hands of emergency physicians) that they merit enumeration in some detail.