Ongoing developments in contrast media for magnetic resonance imaging (MRI) 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 MRI applications. Currently available for clinical use are four low-molecular-weight gadolinium complexes which distribute in the extracellular fluid space and highlight defects in the blood-brain barrier. An estimated 20–45% of patients undergoing MRI examination receive one of these governmental-approved gadolinium complexes. 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.