Ultrasound imaging has also evolved with the latest advancement being acoustic radiation force impulse imaging, which produces elastograms, or images showing the spatial distribution of tissue stiffness. Viscous heating is produced as the oscillating acoustic field causes relative motion between adjacent tissue regions on the scale of the acoustic wavelength. This chapter discusses ultrasound physics and some methods for calculating the ultrasound field. The boundary condition most often employed in ultrasound modeling is that the field only contain a finite amount of energy and hence must tend to zero as distance from the source increases. The chapter discusses some of the tools for predicting linear ultrasound fields. Numerical simulation is a particularly important adjunct to bench studies in the field of therapeutic ultrasound. This is due to the intrinsic limitations of measurement equipment.