Many applications ranging from optical communications through atmospheric science to solar physics require ultra-narrowband optical filtering. The Faraday Anomalous Dispersion Optical Filter (or FADOF) is one method for achieving such filtering. The FADOF applies the Faraday Effect, Zeeman Effect, and anomalous dispersion in an atomic vapor to filter light of a single wavelength with bandwidths on the order of 1 GHz. Uniform transmission approaching 80% over nearly the full bandwidth is achievable, with out-of-band rejection of 40 dB. We have developed such a filter in order to extend lidar measurements of potassium density and mesopause-region temperatures into daylight hours. In this talk, I will describe how the filter works.