Program FILM models multilayer thin film stacks on a substrate. All input and output is via text on screen or files on disk. It calculates many optical properties including reflectance, transmittance, electric and magnetic fields, the Poynting vector, and optical rotation and ellipticity. Materials are characterized by their refractive index or dielectric tensor, so optically active and birefringent materials can be modeled. There is a small library of the wavelength-dependent optical properties of various materials and the user can easily add more. The program generally follows the 2x2 matrix technique of Mansuripur which is described in detail in the accompanying 41 page manual that describes the underlying physical equations upon which the program is based as well as providing many examples, often taken from the literature, demonstrating some of calculations that are possible.