Thioredoxins (Trx) are a ubiquitous family of proteins that modulate the enzymatic activity of their substrate proteins by redox regulation. This is achieved by reduction of a disulfide bond within their target proteins. A conserved pair of cysteine residues in Trx is required for catalysis of the dithiol–disulfide exchange with their target proteins. A single-cysteine mutant capable of forming a stable mixed disulfide bond with target proteins was immobilized on resin and used to capture potential target proteins. By using this method, a number of previously unidentified Trx-target protein candidates were captured from various organisms and organelles. Following the development of this technique, more than one hundred proteins have been reported as potential Trx targets, allowing significant progress to be made in our knowledge and understanding of Trx-target proteins.