Pressure elds generated by single and multiple element transducers are routinely simulated during the initial design of an ultrasound applicator and also throughout the subsequent characterization and optimization of power depositions, temperature distributions, and thermal doses produced by these thermal therapy applicators. Numerical models describe the diraction of ultrasound produced by single transducers, xed-phase multiple transducer congurations, and ultrasound phased arrays. Nonlinear eects are also incorporated into some of these numerical models. e resulting power deposition then provides the input to the bioheat transfer equation. Temperatures are computed for most applications, and thermal doses are also calculated, especially for simulations of ablation therapy. In more advanced models, the eects of intervening tissue heating are considered, beamforming algorithms are evaluated, and more complicated issues such as patient anatomy and tissue inhomogeneities are also included. Patient treatment planning then combines several of these models in an eort to optimize the temperature distribution or thermal dose in the tumor while sparing sensitive normal tissues.