This paper discusses the theory and practice of tunable spectral filters, particularly the types that operate on the principle of light interference and diffraction in a birefringent crystal. Three methods of tuning are described: acousto-optic (AOTF), electro-optical (EOTF) and electromechanical (Solc). Capabilities and limitations of all three are compared. The AOTF is most versatile, is simple to construct, and can be rapidly tuned. Due to its relatively high drive power for mosaic sensor applications AOTF has been limited to a small portion of the focal plane (e.g., for laser warning). In principle, the EOTF has the advantage of low drive power. However, because of the material problems and fabrication complexity involved, practical implementation on the focal plane is probably many years away. For near-term mosaic sensor requirements, the electromechanically tuned Solc appears to be the most promising solution.