The clinical application of contrast-enhanced two-dimensional echocardiography was initially introduced in 1968 and demonstrated significant improvement in visualization compared to non-contrast-enhanced ultrasound. Its use has expanded to multiple vascular territories including the thoracic and abdominal aorta, carotid artery, lower extremity arteries, and venous circulation. Current contrast agents consist of stabilized gas-filled microbubbles (1–7 μm in diameter) that offer adequate safety profiles and improved efficacy. Microbubbles do not diffuse out of the circulation and thus behave as blood pool marker, are able to pass through the capillary pulmonary bed, and are stable enough to achieve enhancement for the duration of the examination. Though detectable with Doppler systems, special multipulse insonating sequences have been developed that selectively display their presence, whether in large vessels or in the microvasculature; this latter vascular beds can now be interrogated for the first time with ultrasound. The effect of microbubbles depends on the fact that gases are compressible, whereas tissue is relatively incompressible. The current chapter focuses on the biology, clinical applications, and future advancements in contrast-enhanced ultrasound. Developments in material engineering and biotechnology will continue to improve the current standards and expectations; a better understanding of the dynamic flow of microvascular beds and focused delivery of biological compounds should enhance end-organ imaging and therapeutic strategies.