Abstract: Iron-sulfur (Fe/S) proteins are involved in a wide variety of cellular processes such enzymatic reactions, respiration, cofactor biosynthesis, ribosome biogenesis, regulation of gene expression, and DNA-RNA metabolism. Assembly of Fe/S clusters, small inorganic cofactors, is assisted by complex proteinaceous machineries, which use cysteine as source of sulfur, combine it with iron to synthesize an Fe/S cluster on scaffold proteins, and finally incorporate the cluster into recipient apoproteins. In eukaryotes, such as yeast and human cells, more than 20 components are known that facilitate the maturation of Fe/S proteins in mitochondria, cytosol, and nucleus. These biogenesis components also perform crucial roles in other cellular pathways, e.g., in the regulation of iron homeostasis or the modification of tRNA. Numerous diseases including several neurodegenerative and hematological disorders have been associated with defects in Fe/S protein biogenesis, underlining the central importance of this process for life. Expected final online publication date for the Annual Review of Biochemistry Volume 77 is June 02, 2008. Please see http://www.annualreviews.org/catalog/pubdates.aspx for revised estimates.