The experimental and theoretical publications on itinerant metamagnetism—the jump-like transition, induced by a magnetic field, of a paramagnetic system of itinerant electrons into a magnetically ordered state—are reviewed. The conditions necessary for the appearance of the metamagnetic transition are formulated on the basis of Stoner's model, and a theory of itinerant metamagnetism for weak itinerant magnets is presented. The effect of fluctuations of the spin density on the properties of an itinerant metamagnet is examined. The results of experimental studies of itinerant metamagnetism, induced by both external and internal effective magnetic fields, in a system of electrons of a number of intermetallides are presented. The systems Co(Se1-xSx)2, Y(Co1-xAlx)2, and Lu(Co1-xAlx)2, the compounds RCO2-1, ThCo5, CeCo5, and others are studied. Some other phenomena associated, like itinerant metamagnetism, with the sharp energy dependence of the density of states near the Fermi level are discussed.