Compact, robust and broadband sources in the terahertz range are extremely important in diverse applications such as spectroscopy, imaging, communication, and radar. A review of the current state-of-the-art is presented with emphasis on Schottky diode based frequency multiplier technology. Frequency multiplier circuit chips fabricated on few micrometer thick GaAs membranes and packaged in low-loss waveguide circuits have demonstrated tens of microwatts of output power up to 1.9 THz. This breakthrough has enabled sensitive heterodyne receivers in the terahertz range that will be flown on an upcoming ESA mission to answer fundamental questions about our universe.