An apparatus for monitoring and controlling the wavelength of laser radiation includes at a least one optical filter for receiving laser radiation and for transmitting and reflecting first and second filtered beams, respectively. The beams are filtered according to respective first and second spectral filter functions that cross at least one crossing wavelength. A beam comparing element compares the intensities of the first and second filtered beams and produces an error signal responsive to the deviation of the wavelength of the laser radiation from a set point wavelength. The beam comparing element can include first and second optical detectors and an error circuit for producing the error signal by taking a ratio or the difference of the signals detected by the detectors. Varying the angle of incidence of the laser radiation upon at least one optical filter varies the spectral filter function of that for selecting or varying the operating wavelength of the laser. A laser wavelength controller can receive the error signal for stabilizing or tuning the wavelength of the laser radiation. Several addition embodiments of the invention are disclosed.