The glutathione- and thioredoxin-dependent reduction systems are responsible for maintaining the reduced environment of the Escherichia coli and Saccharomyces cerevisiae cytosol. Here we examine the roles of these two cellular reduction systems in the bacterial and yeast defenses against oxidative stress. The transcription of a subset of the genes encoding glutathione biosynthetic enzymes, glutathione reductases, glutaredoxins, thioredoxins, and thioredoxin reductases, as well as glutathione- and thioredoxin-dependent peroxidases is clearly induced by oxidative stress in both organisms. However, only some strains carrying mutations in single genes are hypersensitive to oxidants. This is due, in part, to the redundant effects of the gene products and the overlap between the two reduction systems. The construction of strains carrying mutations in multiple genes is helping to elucidate the different roles of glutathione and thioredoxin, and studies with such strains have recently revealed that these two reduction systems modulate the activities of the E. coli OxyR and SoxR and the S. cerevisiae Yap1p transcriptional regulators of the adaptive responses to oxidative stress.