Ultrasound contrast agents (UCAs) consist of gas-filled microbubbles stabilized by a shell of lipid, protein, or polymer, sized to remain within the vascular space. When insonated, microbubbles resonate and produce strong nonlinear echoes detected by contrast-specific imaging modes. UCAs are used to improve endocardial border definition in echocardiography (LVO — left ventricular opacification), characterize focal liver lesions, and evaluate organ perfusion. The most commonly used agent in the US is Optison and Definity. Contrast-enhanced ultrasound (CEUS) is an expanding clinical application.