Gluconeogenesis is reviewed from the standpoints of its functions and relationships to general metabolism, the biochemical mechanisms involved in its control, and its alterations in various physiologic and pathologic conditions. Its functions are discussed in relationship to cerebral metabolism, the Cori cycle, the alanine cycle, and ammonia production by the kidney. The pathways of glucose synthesis from major physiologic substrates are outlined. Control mechanisms for gluconeogenesis, which appear to operate under physiologic conditions, are discussed. Particular attention is paid to regulation of the following processes or enzymes: supply of substrate from peripheral tissues, substrate uptake by the liver, transport of metabolites across the mitochondrial membranes, pyruvate carboxylase, pyruvate dehydrogenase, the Krebs cycle, P-enolpyruvate carboxykinase, pyruvate kinase, fructose-1, 6-diphosphatase, P-fructokinase, glucose 6-phosphatase, glucokinase, phosphorylase, and glycogen synthetase. In addition, the effects of changes in ATP, ADP, AMP, cyclic AMP, the [free NADH] / [free NAD⁺] ratio, Ca⁺⁺, and H⁺ are considered. Changes in gluconeogenesis in various situations are described with particular reference to studies in man. An attempt is made to identify hormonal and other factors responsible for alterations in metabolism. The situations considered include the following: brief and prolonged starvation, diabetes and insulin treatment, fetal and neonatal development, pregnancy and lactation, exercise, obesity, glucocorticoid deficiency and excess, alcohol hypoglycemia, hypoxia, and fatty acid infusion. The effects of certain hypoglycemic agents on gluconeogenesis are also described. Some general conclusions are drawn regarding the possible roles and mechanisms of action of glucagon, insulin, adrenal steroids, the sympathetic nervous system, and fatty acids in the control of gluconeogenesis in vivo. Areas needing further research are outlined.