Ultrasound contrast agents (e.g., SonoVue®, Optison®) consist of gas filled microspheres surrounded by a stabilizing coat. They are smaller than red blood cells and similar to blood in their rheological habit. The microspheres are injected into a peripheral or central vein. They pass lung circulation and disperse into the left heart, coronary arteries and body circulation. A few minutes after administration the low-soluble gas of the microspheres is exhaled. The interaction of the ultrasound beam with microspheres leads to oscillation of the microspheres and thereby amplification of the ultrasound signal. This technique allows enhancement of endocardial border but also visualization of myocardial perfusion and assessment of myocardial blood flow reserve. The latter requires measurement of myocardial perfusion a first time at rest and a second time during hyperemia with adenosine. The quotient of both gives myocardial flow reserve. This parameter is of paramount importance in cardiology to detect coronary artery stenosis and reduced coronary or myocardial flow reserve quantitatively.