The heterodyne laboratory is a development facility for terahertz heterodyne receivers and mixers. The laboratory is equipped with an optically-pumped THz gas laser which covers frequencies from 0.5THz to 6THz developed by the Department “Experimental Planetary Physics”. It is very flexible (i.e. large number of laser lines available, short time to switch between emission frequencies). The THz laser is used for the radiometric calibration of heterodyne detectors (mixers). The high quality of this laser makes the laboratory one of the few places in the world where THz mixers can be accurately calibrated. Besides radiometric calibration, it also permits the geometric calibration of mixers and other THz detectors thanks to its dedicated beam-profile measurement facility. The spectral response of mixers and other types of THz detectors can be characterized with a Fourier transform spectrometer adapted specially for detector calibration. For the integration of detectors, several microscopes and a bonding machine are available together with a wide range of microwave instrumentation to characterize the performance of mixers (network analyzer up to 13.5GHz, spectrum analyzer up to 27GHz, frequency synthesizer up to 40GHz, power meter up to 110GHz, and low-noise amplifiers up to 18GHz). The laboratory is available for external users. For example, the mixers for the high-frequency band of the HIFI instrument on Herschel were characterized in this laboratory. Currently a 0.56 THz heterodyne receiver for planetary atmospheric research (e.g. JUICE) developed by an international consortium in the frame of the EU project TeraComp is undergoing characterization.