Elemental metals such as aluminum and copper are highly opaque and reflective as well as highly conductive, and some are magnetic. In molecular materials cooperative phenomena such as conduction and magnetism require intermolecular interactions without exception. They sometimes produce unpaired electrons/holes from neutral or closed-shell molecules, while they more often cancel out the existing unpaired electrons by making them pairs by strong intermolecular interactions such as dimerization, trimerization, and tetramerization. Molecular self-assembly, such as dimerization and tetramerization, could transform the unpaired electrons into paired and localized states, making the solid lose conduction and paramagnetism. In contrast, the unpaired electrons in a narrow band or a rather localized level have localized spins and generally exhibit magnetism. The qualitatively new features of the photomagnetic conductors and giant photoconductors lie in the sharp wavelength selectivity and the nonlinear dependence on the light intensity in their photoresponse.