For 10 years, it was thought that ultrasound (US) contrast agents could be sufficiently detected and imaged with the conventional imaging techniques, now referred to as fundamental imaging. However, it turned out that fundamental imaging was not sensitive enough to detect the contrast agents in the presence of tissue. New imaging techniques that are based on specific properties of the contrast agents, such as nonlinear and transient scattering, proved to be more sensitive. US contrast imaging modalities used today are fundamental, second harmonic, harmonic power Doppler, and pulse inversion; new modalities, such as release burst and subharmonic imaging are emerging. Second harmonic imaging is still not optimal for perfusion imaging applications. However, in combination with Doppler techniques such as power Doppler, it is one of the most sensitive techniques currently available. A complete understanding of the US-contrast agent interaction is essential for further improvements of current detection methods, and the development of new imaging techniques.