The fabrication of Fabry-Perot Etalon Filters with spacer elements of solid fused silica is described. Typically spacer thicknesses of 120pm are used to provide filters with .05nm to .07nm FWHM at 656.3nm centre wavelength. More complex devices having two coherently coupled cavities are described. The effect of optical system parameters on the spectral performance of the solid etalon filters is discussed and suitable operating parameters are suggested. The performance of solid etalon filters is compared with the birefringent filters of the Lyot type.