Based on the application of coherence theory to neutron scattering a description is given of the propagation of neutrons or x-rays through a reflectometer. Important coherence effects at the sample position are discussed. Further, an outline is given how to determine the measured count rate in a detector on the basis of this method including neutron-polarization effects. It is shown in what way the Born approximation and distorted-wave Born approximation can be used within this theory. An outline is given of the phase-object approximation, describing the specular and diffuse scattering from a surface with large surface structure, extending over the existing capabilities of the distorted-wave Born approximation. The incorporation of neutron-polarization effects enables the detailed discussion of neutron spin-echo coding techniques applied to reflectometry.