Iron, copper, and zinc are all essential nutrients. The electron transfer properties of iron and copper are fundamental to processes such as respiration and photosynthesis. Zinc forms the catalytic center in numerous enzymes and has an important structural role in a wide range of proteins. However, all these metals can be toxic if their levels and distribution are not carefully regulated, as their inappropriate binding may compromise cellular function. The uncontrolled redox activity of iron and copper can also lead to the generation of damaging oxygen radicals. Therefore, organisms maintain cytoplasmic metal concentrations at a nontoxic level that is sufficient for growth. A variety of homeostatic mechanisms have been identified, which include the control of translation and RNA stability by iron-regulatory proteins and the metal-dependent trafficking or degradation of metal transporters (39, 109, 138). This review focuses on the role that metal-responsive transcription factors have in regulating trace metal metabolism. These factors are able to sense changes in metal concentrations and coordinate the expression of genes that are involved in the acquisition, distribution, sequestration, and use of metals. Consequently, the ability to mediate metal-responsive gene expression is an important aspect of metal homeostasis in those organisms that contain these factors.