The theory of waveguide windows has been well known for a long time. Nevertheless, in some recent works, several formulas have been used which lead to different results, especially for window absorption. Since the problem of heat generation (e.g. in high-power windows) often has to be solved by people not specialized in rf- or optical engineering, it seems to be useful to summarize the necessary formulas and to discuss their application. This paper presents a set of consistent equations for reflection, transmission, and absorption of a single waveguide mode, passing through a plane transverse window. Special cases, such as a resonant, an antiresonant, and a sheet window, are discussed. The scattering matrix formulation allows an easy calculation, even of more complex multilayer window structures (e.g. double disk windows or coated disks).