Thioredoxin (Trx) is an inflammation-inducible small oxidoreductase protein ubiquitously expressed in all organisms. Trx acts both intracellularly and extracellularly and is involved in a wide range of physiological cellular responses. Inside the cell, Trx alleviates oxidative stress by scavenging reactive oxygen species (ROS), regulates a variety of redox-sensitive signaling pathways as well as ROS-independent genes, and exerts cytoprotective effects. Outside the cell, Trx acts as growth factors or cytokines and promotes cell growth and many other cellular responses. Trx is also implicated in tumorigenesis. Trx is a proto-oncogene and is overexpressed in many cancers and correlates with poor prognosis. Trx stimulates cancer cell survival, promotes tumor angiogenesis, and inhibits both spontaneous apoptosis and drug-induced apoptosis. Inhibitors targeting Trx pathway provide a promising therapeutic strategy for cancer prevention and intervention. More recently, data from our laboratory demonstrate an important role of Trx in expanding long-term repopulating hematopoietic stem cells. In this chapter, we first provide an overview of Trx including its isoforms, compartmentation, and functions. We then discuss the roles of Trx in hematologic malignancies. Finally, we summarize the most recent findings from our lab on the use of Trx to enhance hematopoietic reconstitution following hematopoietic stem cell transplantation.