A review is given of the magnetic and magnetoelastic behavior of both classical and novel materials having Invar characteristics. Contrary to earlier views many materials having such properties exist which contain no iron and have a fairly homogeneous magnetization. Since these materials may be prepared with different Curie temperatures depending on the alloying content, in principle it is possible to tailor-make Invars with any desired properties. The results of the various experimental investigations on these substances, both classical and novel, are described. They are shown to be reasonably consistent with the view that they are weak itinerant ferromagnets. This view allows the consistent application of the Landau-Ginzburg theory in which all coefficients are given explicitly in terms of energy band and itinerant electron interaction parameters. A very brief review is also given of the application of Invar and related substances to a variety of devices.