An internal flat field system provides reference flat field images over the spectral range of WFPC2. These are provided by a "calibration channel" optical system mounted outside the main shroud of WFPC2. The system consists of a series of lamps and diffusers, and a flip mirror which directs the beam into the WFPC2 entrance aperture. The lamp set contains Tungsten incandescent lamps with spectrum shaping glass filters and a Deuterium UV lamp. The flat field illumination pattern is fairly uniform for wavelengths beyond about 1600Å. Short of 1600Å the flat field is distorted due to refractive MgF2 optics. In practice, the flat fields used routinely to calibrate WFPC2 data have been generated by combining flats taken with an external stimulus in thermal vacuum testing with Earth "streak" (unpointed) flats to give the low frequency terms in the OTA illumination pattern. The calibration channel is used primarily to check for internal instrumental stability.