In 1997 and the first half of 1998, numerous publications appeared reporting studies of cofactors and their analogues in classical model systems and in enzyme-catalyzed reactions directed at understanding the enzymatic reactions of their natural cofactors. Model systems based on flavins have provided new insights into enzymatic modulation of the flavin reduction potential, and enzymatic reactions of coenzyme A analogues and derivatives have been employed in several studies of coenzyme A utilizing enzymes. Coenzyme B12 analogues have been utilized as alternate cofactors for B12-utilizing enzymes, while pyrroloquinoline quinone esters and analogues have been employed in model studies of the reactions of quinoprotein-catalyzed reactions.