This review deals with optical properties of Langmuir films formed by successive transfer of monomolecular layers of surface-active organic compounds from the surface of water onto a solid substrate. This gives rise to new solid-state systems consisting of stacks of weakly coupled molecular layers; the properties of molecules (optical, electrical, photochemical, etc.) are not subject to any constraints. The review deals with the properties of monolayers on the surface of water, with their transfer onto solid substrates, and with their electrical, optical, photoelectric, and electrooptic properties. The review concludes with a discussion of applications of Langmuir films in molecular electronics, which is now becoming a new technology.