In contrast to extracorporeal systems, endocavity and catheterbased ultrasound devices have been developed for delivering hyperthermia and thermal ablation from placement within the body. ese minimally invasive ultrasound techniques can be used to apply hyperthermia as an adjunct to radiation therapy and/or chemotherapy, or as a surgical alternative for tumor or tissue ablation. For these applications, the heating source is positioned directly within or adjacent to a deep target volume via placement within a body cavity, lumen, or by direct insertion. Even though these technologies are more invasive than extracorporeal systems, many of these approaches may be preferable for sites where energy localization from external devices is dif-cult or where localization of all power and energy propagation within the target tissue is critical.