Amide nitrogen from glutamine is a major source of nitrogen atoms incorporated biosynthetically into other amino acids, purine and pyrimidine bases, amino-sugars, and coenzymes. A family comprised of at least sixteen amidotransferases are known to catalyze amide nitrogen transfer from glutamine to their acceptor substrates. Recent fine structural advances, largely as a result of X-ray crystallography, now provide structure-based mechanisms that help to explain fundamental aspects of the catalytic and regulatory interactions of several of these aminotransferases. This chapter provides an overview of this recent progress made on the characterization of amidotransferase structure and mechanism.