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The feasibility of High Intensity Focused Ultrasound (HIFU) to noninvasively ablate tumors deep within the body has been investigated in several studies. HIFU is perceived to have the potential to revolutionize the way cancer therapy is performed. However, for treatment effectiveness and patient safety during HIFU therapy it is necessary to accurately predict the thermal field generated by the HIFU transducer in order to decide the optimal energy dosage. Hence there is a need to develop accurate and reliable methods to characterize the induced thermal field. This dissertation presents improved methods to characterize the HIFU thermal field in tissue mimicking materials or excised tissues. In the first phase of the study, the temperature rise is measured by an invasive method, using thermocouples embedded in a vascularized tissue mimicking material subject to direct HIFU sonication at intensity levels between 495 to 1192 W/cm2. However, significant inaccuracies due to thermocouple artifacts generated by interference of the thermocouples with the beam and manual errors in positioning the beam on the thermocouple junctions are detected in the measured data. These inaccuracies are assessed and removed by a computational algorithm developed in the second phase of the study. The refined data is useful in assessing the influence of blood flow on HIFU thermal effects, an investigation relevant to real clinical situations. In view of inaccuracies due to artifacts associated with direct sonication of thermocouples, in the second phase of the study, a new non perturbing method called the inverse heat transfer method is developed. The thermal field is assessed without having to focus the beam directly on thermocouple junctions to prevent artifacts. The temperature rise measured by thermocouples that are remote from the beam is used as an input to the inverse heat transfer algorithm. The algorithm uses an iterative approach to calculate the location as well as angular orientation of the beam with respect to the thermocouple array. Determination of the beam location enables prediction of the artifact-free temperature rise at desired locations within the tissue phantom with a 4-6% accuracy of measurements. In the third phase of the study, MRI-monitored HIFU ablations are performed on freshly excised porcine liver samples. In contrast to the method using thermocouples, the MRI thermometry method is used to non invasively measure the spatial and temporal temperature rise and size of the HIFU lesion or volume of destroyed tissue at different sonication times, (20, 30 and 40 s) and intensities upto 1700 W/cm2. The experimental results are within 10% agreement with the temperature rise and lesion size calculated by solving the acoustic and bio heat transfer equations, thus validating the computational method. Analysis of results obtained from the study lead to the conclusion that the MRI thermometry and inverse heat transfer methods are both convenient and improved methods for assessing the HIFU induced thermal field compared to the conventional method of direct sonication of thermocouples embedded in the tissue medium. In future studies, the inverse heat transfer method may be extended to determine parameters like the acoustic intensity generated in the tissue medium as well as the tissue absorptivity. Also the computations can be modified to account for phenomenon like boiling, cavitation and non linearity to improve accuracy of predictions at higher intensities.