Ultrasound contrast agents have been used for many many years in cardiology. Their application in other fields is more recent, related to the availability of agents that can pass through the pulmonary circulation. Physical properties of contrast microbubbles are closely bound to their gas content, shell composition, frequency of ultrasound beam, pulse repetition frequency, acoustic power and wave phase. Specific ultrasound sequences have to be used to adequately exploit microbubble-specific interactions with the ultrasound beam. The aim of this paper is to review physical properties of ultrasound contrast agents, present imaging sequences developed for optimal use of these contrasts and general applications.