Humans are largely dependent upon glucose-and to a lesser extent, the monosaccharides galactose and fructose-to provide fuel for cellular metabolism. Levels of blood glucose are maintained fairly precisely through a balance of dietary intake, breakdown of established glycogen stores, and gluconeogenesis. Given the importance of normal metabolism of these carbohydrates, it is readily understandable that abnormalities in monosaccharide metabolism are associated with significant disease processes. This section will review the inborn errors associated with altered metabolism of galactose and fructose, as well as the more common disorders associated with defects in glucose metabolism.