Successful clinical applications of contrast agents involve an understanding of the physical interaction of ultrasound (US) with contrast agents. This paper reviews the physical phenomena involved in these interactions and discusses the relevant theoretical background for modeling US-contrast agent interactions. Measurement techniques using US to obtain information regarding contrast agents are summarized. Illustrative clinical applications are given in the second part of the paper. Recent developments in nonlinear imaging techniques and transient techniques are reviewed. New methods, such as depletion perfusion measurement, and high-frequency applications are included.