Lipid-coated, gas-filled submicron-sized bubbles (SMBs) were developed in our laboratories in the early 1990s as an ultrasound contrast agent for enhancing vascular images. Perfluorocarbon gas-water interfaces are very efficient at reflecting ultrasound waves. In the presence of ultrasound frequencies (1–10 MHz) gas bubbles of size 5_m or less, when subjected to the compression and expansion force of a sound wave, resonate and become transmitters themselves. The ultrasound backscatter generated by these SMBs in vivo provides real time images of blood flowing through heart chambers and blood vessels. In addition, these SMBs undergo oscillations and cavitation. The presentation will cover examples of diagnostic applications and potential therapeutic use.