This review summarizes current knowledge about thioredoxin and related topics, with emphasis on the best-characterized systems. The literature on thioredoxin covers a fascinating variety of biochemical systems in which thioredoxin is implicated in functional roles. As introduction to a more detailed description of thioredoxin, a historical perspective is useful. Thioredoxin was first isolated in 1964 and characterized as the hydrogen donor for the enzymatic synthesis of deoxyribonucleotides by ribonucleotide reductase in E. coli (1). The name thioredoxin was assigned to the small protein since its function was dependent on the cyclic reduction-oxidation of a single SS-group (1). The SS-bond of oxidized thioredoxin (thioredoxin-S2) is reн duced by NADPH and a specific enzyme called thioredoxin reductase (9): thioredoxin reductase