As the role for diagnostic and therapeutic contrast-enhanced imaging increases, review of the epidemiology, mechanisms, risk factors, and pretreatment for radiocontrast-mediated anaphylactoid reactions becomes more and more pertinent. Ongoing research has failed to elucidate the precise mechanisms of both early and late reactions, though the current data point to a multifactorial pathogenesis. The risk of reactions has decreased over time as contrast media have evolved from ionic, high-osmolality to nonionic, low-osmolality formulations; however, the expense of the low-osmolality agents limit their universal use. Today, 1-12% of patients exhibit adverse responses ranging from mild to severe, with individual risk depending on the type of contrast administered and certain baseline patient characteristics. For those high-risk patients who must receive contrast, effective pretreatment guidelines have been established.