Ultrasound contrast agents may modify any of the three basic physical interactions between ultrasound and body tissues: absorption, reflection and refraction, and then the amount of backscatter can be increased. New imaging techniques including second harmonic imaging, interval-delay harmonic imaging, pulse-inversion harmonic imaging and sub-harmonic imaging proved to increase the sensitivity of altrasound contrast agents. There are several types of agents that have been used for liver imaging: free gas bubbles, saccharide microparticle suspensions, encapsulated microbubbles, perfluorocarbons, organ or tumor-specific agents. Ultrasound contrast agents can show characteristic vascular features of liver, specific types of hepatic tumors, or characteristic dynamic phases of perfusion imaging; therefore they can provide meaningful information for clinical diagnosis.