A non-invasive technique for the localization and evaluation of thermal effects in tissue due to focused high intensity ultrasound beams is described in this paper. A combined therapy/imaging transducer assembly is used to deliver a specific dose (ultrasound time-intensity product) to a tissue sample and to gather A-mode data as a function of time. In-vitro experiments performed on calf liver and in-vivo experiments performed on rat brains and liver have shown the following: 1. Reflection coefficients calculated based on a low-order autoregressive (AR) tissue model assumption and signal entropy follow the temperature rise at the heating point; 2. Heated region size and position can be monitored with high accuracy. Such information could be used effectively for feedback purposes during a rapid heating hyperthermia or ultrasound surgery procedure.