A complete survey of papers published in 1945 was made difficult by the inaccessibility of many European journals. With few excep tions, a review. of the carbohydrate metabolism of microorganisms could not be included because of the limitation of space.The increasing importance of isotopes as a tool in biochemistry is reflected in the rapid advances which have been made in inter mediary carbohydrate metabolism with this technique. Isotopes have also been used to great advantage in demonstrating reversibility of enzymatic reactions, particularly those involving carbon. dioxide fixa tion or phosphate transfer. The common metabolic pool for carbon fragments derived from the three principal foodstuffs and their com mon oxidative pathway has made it impossible to discuss the inter mediary metabolism of carbohydrates without some reference to that • of the other foodstuffs.The gap which often exists between results obtained in experi ments on the whole animal and on isolated enzyme systems is being bridged to some extent. Here again the use of isotopes plays a lead ing role, but a unified picture will not emerge until the mechanism of the regulatory effect of hormones on enzyme systems is more clearly understood.The usual abbreviations have been used: ATP and ADP for adenosinetriphosphate and adenosinediphosphate ; TPN and DPN for tri-and diphosphopyridine nucleotide; and P for phosphate.