Ultrasonography has, until recently, lacked effective contrast-enhancing agents. Micrometer-sized gas bubbles that resonate at a diagnostic frequency are ideal reflectors for ultrasound. However, simple air bubbles, when injected into the blood stream, disappear within seconds through the combined effects of Laplace pressure, blood pressure, and exposure to ultrasound energy. Use of fluorocarbon vapor, by extending the persistence of microbubbles in vivo from seconds to minutes, propelled contrast ultrasonography into clinical practice. Imaging techniques that selectively suppress tissue, but not microbubble signal, further increase image contrast. Approved products consist of C3F8 or SF6 microbubbles, and N2 microbubbles osmotically stabilized with C6F14. These agents allow the detection and characterization of cardiovascular abnormalities and solid organ lesions, such as tumors. By providing higher quality images, they improve the accuracy and confidence of disease diagnosis, and can play a decisive role in clinical decision making. New objectives include agents that target specific cells for the molecular imaging of disease, and drug and gene delivery, including ultrasound-triggered delivery.