A wavelength monitoring system for continuously monitoring the wavelengths of different optical channels transmitted through a wavelength division multiplexed (WDM) fiberoptic system. In one construction, an optical diffraction grating is used to disperse the light being monitored onto a rotating polygon having mirrors on the facets thereof, such that the rotating polygon reflects the light onto a detector located behind a slit. In a second construction, the light is projected onto a rotating, hollow transparent cylinder having a thin film filter deposited on a surface thereof, wherein the thin film filter comprises a Fabry-Perot structure which has a gap which varies as a function of its position on the rim of the cylinder, such that the varying gap allows for a varying transmission wavelength which varies as a function of angular position. A detector is positioned in the interior of the cylinder to detect light passing through the rotating thin film filter.