This article begins with a brief introduction to Fabry-Pérot interferometers and goes on to describe the use of Fabry-Pérot étalons as narrow-band filters. Properties of solid étalons with fused silica, mica, and lithium niobate substrates are discussed and compared with those of the familiar Lyot birefringent filter. Examples of applications of étalon filters in high-resolution solar imaging and vector magnetography are given. Recent measurements on an experimental étalon made with Y-cut lithium niobate are reported. Each passband of the experimental étalon transmits 80% of linearly polarized light (40% of unpolarized light), and the voltage-tuning parameters are better than for the Z-cut étalon. For the o-ray passbands, only ± 1900 V is required to tune over one free spectral range, vs ± 4000 V to tune a Z-cut étalon. When used with conventional thin-film filters for order selection, the étalon filter can be tuned to any wavelength from 0.4 to 4.0 μm.