Thioredoxins (Trx) are members of an evolutionarily conserved family of redox-active proteins containing a conserved active site dithiol motif. Trx supports diverse reduction reactions, including several of direct toxicologic interest, but relatively little information is available concerning the roles of Trx under specific toxicologic conditions. Accumulating evidence suggests that Trx serves a partially overlapping and highly complementary role to the glutathione (GSH) system in protecting against toxicity. GSH and Trx both function in the reduction of peroxides through the action of multiple GSH peroxidases and Trx peroxidases (peroxiredoxins), respectively. However, GSH is a small molecule that is present at millimolar concentrations, thereby providing a potential mechanism for elimination of alkylating electrophiles. In contrast, even though Trx is only present at micromolar or submicromolar concentrations, its dithiol motif makes it suited to reverse oxidative changes to proteins, including reduction of protein disulfides, methioninyl sulfoxides, and cysteinyl sulfenic acids. Moreover, Trx functions in redox-sensitive signal transduction, transcriptional activation of stress response genes, ribonucleotide reduction in synthesis of deoxyribonucleotides for DNA repair, and post-injury cell proliferation. Molecular studies show that the predominant cytoplasmic/nuclear form, Trx-1, and the mitochondrial form, Trx-2, both protect against oxidative stress, that both are essential for embryonic development, and that Trx-1 is inducible in response to oxidative stress. Because of the differences between GSH and Trx in distribution, catalytic activities and reactivities with electrophiles, particularly with the important role to be played by glutathione S-transferases, considerable research is needed to clarify their respective roles in protection against specific toxicologic conditions.