The Fabry–Perot interferometer has become a standard spectroscopic tool in scientific laboratories for the study of Brillouin scattering. This article examines some basic properties of the Fabry–Perot interferometer with the student or beginning user in mind in order to demonstrate why it is especially useful for this application. Single and complex Fabry–Perot systems in use for Brillouin scattering studies are discussed starting from elementary equations to show, in particular, how the half‐width of the transmission function depends upon the instrumental configuration. The primary results are summarized in graphical form. ©1996 American Institute of Physics.