Up to the present time no image-forming depolarizing filter has been described which will work for monochromatic light. This paper describes a simple monochromatic depolarizer. This depolarizer consists of two variable wave plates whose axes are separated by an angle of 45°. The retardation of the first wave plate varies from zero to 2π. The second wave plate varies from zero to 4π. This variation is made to occur in a time which is short compared to the time constant of the apparatus in which the depolarizer is used. Typical components are Z cut plates of ammonium-dihydrogen-phosphate. The retardation of these plates can be made to vary by applying an electric field along the optic axis.A discussion is also given of the Lyot depolarizer which is effective for light which covers a wide wavelength range. Tolerances are derived for the behavior of the Lyot depolarizer and experimental measurements are shown of its behavior.