Ongoing developments in contrast media for magnetic resonance (MR) imaging should lead to an improvement in sensitivity for the detection of disease, better definition of normal and pathologic anatomy, added functional information, and an expansion of diagnostic MR imaging applications. Currently, only gadopentetate dimeglumine, a contrast enhancer of the extracellular fluid space that is able to show defects in the blood-brain barrier, is governmentally approved for routine clinical use in North America. An estimated 25%-35% of patients undergoing MR imaging receive this drug. Multiple, potentially competitive formulations have been tested and may soon be approved. In addition, contrast agents differing in purpose and primary magnetic effect, both paramagnetic proton relaxation and magnetic susceptibility agents, are being developed. These include agents for enhancing the blood pool, myocardium, liver, lymph nodes, tumors, and gastrointestinal lumen. Criteria for suitability of new contrast agents include diagnostic efficacy, safety, stability, pharmacology, and cost.