Gadolinium (Gd)-based paramagnetic contrast agents are relatively safe when used in clinically recommended doses. However, with the rapidly expanding body of literature linking Gd-based paramagnetic contrast agents and nephrogenic systemic fibrosis (NSF), awareness of the potential side effects and adverse reactions from Gd is now an important requirement for practicing radiologists. In addition to the ongoing accumulation and analyses of clinical NSF data, it is also essential for the practicing radiologist to understand the biochemical characteristics of the extracellular Gd-chelates. The purpose of this review is to consolidate and update the available information on known side effects, adverse reactions, and toxicity of the Gd chelates, with particular emphasis on the potential mechanisms of NSF. J. Magn. Reson. Imaging 2007;26:1190–1197. © 2007 Wiley-Liss, Inc.