Radiographic procedures which require the intravascular administration of water-soluble radiocontrast media are performed with increasing frequency. Each examination carries risks that are related either to the technique itself or to the opaque medium chosen. The pathogenesis of radiocontrast media-related adverse effects cannot be explained by a unique theory. The major factors implicated are direct chemotoxic effects and the physicochemical properties of contrast media, the latter being the basis for development of new contrast agents. With nonionic opaque media cardiovascular adverse effects, heat sensation and local pain are observed less frequently. However, it remains unclear whether the incidence of organ dysfunction or anaphylactic reactions with nonionic contrast media currently used can be reduced. This review compares ionic and nonionic contrast media, and current thoughts on the pathophysiology and treatment of adverse reactions are presented.