Ultrasound contrast agents contain tiny gas-filled microbubbles which on intravenous injection are transported like red blood cells by the bloodstream. They are distributed to the entire macro- and microvascular system without extravasation (blood-pool tracer). Contrast-specific ultrasound modalities allow selective imaging of these microbubble contrast agents in real time, with excellent separation from underlying tissue signals. The dynamic wash-in and wash-out of a contrast bolus allows the detection and quantitative assessment of vascular flow and parenchymal perfusion. On complete filling of vascular structures the microbubble distribution delineates the vascular lumen and depicts the vascular architecture, down to tiny branches within parenchymal tissue. Mathematical models for bolus kinetics and flash/replenishment kinetics allow precise quantification of relative blood volume and blood flow, with high temporal and spatial resolution. Contrast-enhanced ultrasound examinations can be repeated at short intervals and performed at the bedside, making this diagnostic method convenient for guidance of interventions and therapy monitoring. Future applications include the use of microbubbles as target-specific molecular imaging agents and as delivery systems for local drug and gene therapies.