Program GRATING models multilayer conformal thin film stacks on a diffraction grating. Input and output is via text to the screen or files on disk. It calculates various optical properties including reflectance and transmittance of the diffracted orders as a function of polar and azimuthal angles of incidence, and the electric and magnetic fields through the film stack. Materials are characterized by their scalar refractive index. The program follows the method of Chandezon 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. It is particularly useful for sinusoidal gratings. It can also handle skewed and tilted gratings and some trapezoidal gratings, but is not suitable for square gratings.