In recent years, the imaging-based diagnosis of mass liver lesions has become increasingly complex due to the number and morphological variability of lesions that modern imaging techniques are currently able to display. If the sensitivity in detection has greatly increased, characterisation has remained difficult and represents a critical challenge for the clinician. The availability of blood-pool contrast agents for ultrasound (US), in particular second-generation US contrast agents based on perfluorocarbon- or sulfur-hexafluoride-filled microbubbles, and the development of contrast-specific software and technologies have opened up new perspectives both for the immediate characterisation of any mass lesion detected in the liver and for increasing the sensitivity of US in the detection of liver metastases. Taking into account the great impact of this new technology on clinical practice, the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) organised, in January 2004, in Rotterdam, a consensus meeting of experts in order to develop guidelines for the use of US contrast agents in the diagnosis of liver diseases . These guidelines, as well as discussions of further advances in the clinical application of contrast-enhanced harmonic US are presented in this book by an internationally renowned group of experts. The book represents provides an important starting point for clinical implementation of this new diagnostic procedure.