Abstract: We review macroscopic properties of materials that can be modulated by light through switching between the two stable states of a photochromic system. In special cases, the photochromic compound is the only component of the material, but in most cases it is either embedded or covalently linked to a suitable host, which is normally a liquid crystalline phase, a glassy or a rubbery polymer, or an interface. We analyze examples that illustrate changes in refractive index, transmission of polarized light, reflectivity, light dispersion, polarization, charge transport capability, dielectrical properties, stable phase, microscopic surface relief in large areas, hydrophobicity of the surface, permeability, and even bulk mechanical deformations. The basis of such changes at a molecular level is associated with a difference in the shape, dipole moment, polarizability, or electronic features of the photochromic species, and their different interaction with the environment. In some cases, studies were performed at the single molecule level. Each effect is discussed stressing the favorable features of the particular photochromic compounds to induce the changes and its interaction with the environment that make it suitable for the appointed application. [Copyright &y& Elsevier]