Fe-S clusters are one of the most ancient and ubiquitous prosthetic groups in biology. The range of their biological functions makes them one of the most versatile prosthetic groups as well. While the last twenty years have provided a wealth of information on the functional aspects of Fe-S clusters, very little is known about how organisms deal with the delivery of iron and sulfide, or the manufacture and insertion of Fe-S clusters into apoproteins. Recent studies have shown that the movement of metals into and through cells is a tightly regulated process, and many aspects of this metal homeostasis are conserved throughout the three kingdoms of life. One of the most conserved process in metal homeostasis in particular and in biology in general, is the assembly of iron-sulfur clusters. Research in this thesis centers around two proteins, NifU and NifS, and their more general homologues, IscU and IscS. A combination of molecular biology, biochemistry, and spectroscopy detail a process in which NifS or IscS direct the assembly of transient iron-sulfur clusters upon NifU or IscU. The spectroscopic and biochemical properties of NifU and IscU are consistent with a role in providing a scaffold for the assembly of transient iron-sulfur clusters that are transferred intact to different apoproteins. In a complementary study, NifEN, which is implicated as a scaffold for the assembly of the heterometallic Mo-Fe-S cluster that constitutes the active site of nitrogenase, has also been characterized.