A comprehensive discussion of the resolution function in specular and diffuse X-ray reflectivity is given. This is particularly relevant due to the proliferation of this technique in the field of liquid and solid surfaces and multilayer systems. A simple quantitatively correct interpretation of the diffuse reflectivity is possible if the resolution function is separable for the two directions in the scattering plane. This can be accomplished using a symmetric resolution set-up and specific types of scans (so-called radial scans and rocking scans).