Although ultrasound is considered to be a very safe modality, there is a small potential risk of doing damage to tissues, referred to as bioeffects. Because of this risk, practitioners must make prudent use of ultrasound. Obviously, in order to be prudent when applying ultrasound, an understanding must be developed of the mechanism by which bioeffects occur, as well as the parameters which most directly impact the likelihood of bioeffects. In this module we integrate previously reviewed concepts together to formalize our understanding of the two primary bioeffect risks, mechanical bioeffects, related to cavitation, and thermal bioeffects, related to temperature increase. Additionally, in this module we review some of the AIUM statements as regard the safety of ultrasound and the risk of bioeffects.