Reactive oxygen species (ROS) are generated as toxic by-products of aerobic metabolism, but are also essential biomolecules in cell signaling. The thioredoxin (Trx) system is a major enzymatic system modulating ROS levels and is important for redox regulation of cellular function. It consists of Trx and thioredoxin reductase (TrxR), which reduces Trx using NADPH. Most, if not all, of the functions of Trx depend on the activity of TrxR. Mammalian TrxR enzymes are selenoproteins with broad substrate specificities, and alteration of cytosolic TrxR1 expression and activity is likely to be an important determinant for the control of cellular redox regulation. TrxR1 activity in cells seems to be modulated by an intricate interplay, involving a housekeeping type promoter in combination with alternative splice variants and transcriptional start sites, posttranscriptional regulation through AU-rich elements, inactivation by electrophilic agents and by itself modulating the effects of several key signaling molecules. TrxR1 activity is also intimately linked with several aspects of selenium metabolism, and hence selenoprotein function in general. Here, we summarize the current knowledge of these different levels of TrxR1 regulation in diverse cell types and in response to growth and signaling events.