The perceived importance of tellurium (Te) in biological systems has lagged behind selenium (Se), its lighter sister in the Group 16 chalcogens, because of tellurium's lower crustal abundance, lower oxyanion solubility and biospheric mobility and the fact that, unlike Se, Te has yet to be found to be an essential trace element. Te applications in electronics, optics, batteries and mining industries have expanded during the last few years, leading to an increase in environmental Te contamination, thus renewing biological interest in Te toxicity. This chalcogen is rarely found in the nontoxic, elemental state (Te0), but its soluble oxyanions, tellurite (TeO32-) and tellurate (TeO42-), are toxic for most forms of life even at very low concentrations. Although a number of Te resistance determinants (TelR) have been identified in plasmids or in the bacterial chromosome of different species of bacteria, the genetic and/or biochemical basis underlying bacterial TeO32- toxicity is still poorly understood. This review traces the history of Te in its biological interactions, its enigmatic toxicity, importance in cellular oxidative stress, and interaction in cysteine metabolism. © 2009 Federation of European Microbiological Societies. Published by Blackwell Publishing Ltd. All rights reserved.