Gadolinium (Gd) released from Gd-based MR contrast agents is the likely etiologic agent of nephrogenic systemic fibrosis (NSF). The biodistribution and kinetics of both chelated Gd and free Gd(3+) are reviewed, including demonstrations of Gd in tissues of NSF patients. The purpose of this review is to consolidate the available information on distribution, metabolism, excretion and retention of Gd with particular emphasis on the possible mechanisms in the development of NSF. We also raise serious concern about the growing tissue burden of Gd with the release of Gd from its chelates and its yet unknown consequences. The development of NSF as well as potential long-term consequences of tissue Gd retention represent a part of the spectrum of Gd toxicity.