Ultrasound contrast agents (UCAs) are shell encapsulated, gas-filled microbubbles developed originally for ultrasound imaging enhancement. UCAs are approximately 1–10 micrometers in diameter with a shell typically comprised of lipid, protein, or polymer. When injected into the bloodstream, the high compressibility of these microbubbles, relative to the surrounding blood and tissue, and their highly nonlinear response to ultrasound, leads to strong enhancement of the blood-tissue contrast in the resulting ultrasound image. While UCAs have been commercially available since the early 1990’s [1] for ultrasound imaging, they are more recently being exploited for therapeutic applications, for example, as vehicles for drug delivery and gene therapy, and thermal and mechanical tissue ablation. The effectiveness of UCAs in therapeutic applications depends strongly on the nonspherical character of the bubble oscillation, which can effect the breakup and release of therapeutic agents from the UCA, as well as the formation of high-speed jets near the tissue interface. In this work, two different models for nonspherical oscillation of UCAs are presented: one for small shape oscillations of a lipid-coated bubble, and one for large nonspherical oscillations of a polymer-coated bubble. Nonspherical shape mode stability and dynamics are investigated with each model for ranges of ultrasonic frequency and amplitude relevant to medical applications.