mong the factors that have contributed to the rapid growth in the medical use of diagnostic ultrasound (DUS), an important one is the reputation for safety that it enjoys. This reputation did not come about immediately, and it has not been maintained without effort. Instead, the reassurance that users and patients can feel about the safety of DUS comes from years of research into the conditions under which hazards can be avoided and application of the findings to guidelines for equipment manufacture and medical practice. Efforts of this kind that have been made by individuals in many countries as well as by national and international organizations have been reviewed recently.1,2 This article emphasizes, in particular, the contributions of the American Institute of Ultrasound in Medicine (AIUM) to these efforts; it is part of a series of articles on various topics prepared in celebration of the AIUM’s 50th anniversary. In the earliest days of the AIUM, its members were involved mostly with applications of ultrasound in physical therapy; at the 1957 AIUM meeting, only 1 of 35 papers dealt with the possibility of using ultrasound for diagnosis. Thus, it was well known in the formative years of the AIUM that exposure of a patient to ultrasound is capable of producing physiologic change. In a therapeutic application, the aim, of course, is to produce a change for the better. It was known, though, from research with laboratory animals, that harm can be produced by ultrasound under some conditions, for example, as described by Lehmann3 (Fig. 1). As diagnostic techniques improved, the use of DUS increased rapidly, and by the late 1960s, the AIUM was concerned primarily with DUS applications.4 As this occurred, it was recognized that technical advice should be provided for avoiding harm from the use of this new modality and for increasing its effectiveness. For these purposes, the AIUM established committees, including those leading to the present Bioeffects Committee and Technical Standards Committee. The Bioeffects Committee was asked to provide information to the membership on matters relating to the biological effects of ultrasound, especially when these matters relate to the safety of clinical ultrasound. From time to time, the Bioeffects Committee produced consensus statements on important matters. The first of these (later to be modified) was included in communications from the committee in 19765 and, after some revision, in 1978.6 An excerpt from the 1978 form of the statement is shown in Figure 2; accompanying this was text specifying the intensity referred to as the spatial-peak temporal-average (SPTA) value. Also, it was warned that the statement was based on all seemingly reliable data (from animal experiments) available at the time, although such data for operating conditions Abbreviations AIUM, American Institute of Ultrasound in Medicine; DUS, diagnostic ultrasound; MI, mechanical index; NCRP, National Council for Radiation Protection and Measurements; SPTA, spatial-peak temporalaverage; TI, thermal index