Our mechanistic understanding of the conversion of vitamin B-12 into coenzyme B-12 (a.k.a. adenosylcobalamin, AdoCbl) has been substantially advanced in recent years. Insights into the multiple roles played by ATP:cob(I) alamin adenosyltransferase (ACA) enzymes have emerged through the crystallographic, spectroscopic, biochemical, and mutational analyses of wild-type and variant proteins. ACA enzymes circumvent the thermodynamic barrier posed by the very low redox potential associated with the reduction of cob(II) alamin to cob(I) alamin by generating a unique four-coordinate cob(II) alamin intermediate that is readily converted to cob(I) alamin by physiological reductants. ACA enzymes not only synthesize AdoCbl but also they deliver it to the enzymes that use it, and in some cases, enzymes in which its function is needed to maintain the fidelity of the AdoCbl delivery process have been identified. Advances in our understanding of ACA enzyme function have provided valuable insights into the role of specific residues, and into why substitutions of these residues have profound negative effects on human health. From an applied science standpoint, a better understanding of the adenosylation reaction may lead to more efficient ways of synthesizing AdoCbl.