In many antiferromagnets the elementary magnetic cell contains equal numbers of the same magnetic moments aligned in opposite directions. The theory of antiferromagnets and ferrimagnets, based on the sublattice model, can be constructed in a way analogous to the continual theory of ferromagnets. The static and dynamic magnetic susceptibilities of an antiferromagnet are small and (in single crystals) highly anisotropic. The exchange interaction in antiferromagnets and ferrimagnets, as well as in most ferromagnets, is mainly indirect. In nonmetallic crystals the indirect exchange interaction via anions plays the main role. The effective exchange integral of this interaction is negative, which leads to antiparallel orientation of the magnetic moments of these ions. Other exchange interactions may be either ferromagnetic or antiferromagnetic. In ferrimagnets, the steady sublattice magnetizations not only differ from each other in magnitude but can differently depend on composition and temperature.