This paper describes the performance characteristics of an electronically tunable optical filter based on the electro-optic effect. Like the acousto-optic tunable filter, this device operates by selectively coupling principal polarizations in a birefringent crystal at a phase-matched wavelength by means of a spatially periodic refractive index perturbation. Instead of a traveling acoustic wave, however, the electro-optic tunable filter employs a temporally static electric field. The main advantages of this filter are its very low power consumption and its versatility of passband programming, by virtue of sepa-rately addressable voltages under microprocessor control. Two basic embodiments of the filter structure are described, one using longitudinal fields collinear with the light beam, the other using transverse fields in a "thick waveguide" configuration. Various examples of passband synthesis are described that result from apodizing the amplitude of the applied electric field perturbations. Experimental results of filter transfer characteristics at visible wavelengths are presented for the basic transfer function, for passband broadening, and for sidelobe suppression, together with theoretical computer plots showing excellent agreement. Typical driver voltages are 50 to 100 V.