The purpose is to make clear the fundamental physical nature of the Invar problem, disregarding its details. After a brief discussion of the practical applications of the Invar alloys, the large magnetovolume effects and the anomalous magnetic properties are reviewed. The basic ideas of the various interpretations of the Invar effects, as well as the microscopic properties of the Invar alloys, are summarized. The physical background of the Elinvar alloys is also presented. Recent observations of Invar-type effects in new materials suggest that the Invar effect is rather common in itinerant ferro- and antiferromagnets.