Biosynthesis of MoFe protein and, particularly, that of its associated P-cluster and FeMoco has raised a significant amount of interest because of the biological importance and chemical exclusiveness of these unique clusters. Following a brief introduction to the properties of Azotobacter vinelandii MoFe protein, this chapter will focus on the recent progress toward understanding the assembly mechanism of MoFe protein, with an emphasis on studies that provide important structural or spectroscopic insights into this process.