The frequency range between 100 GHz and a few THz has been used in fundamental research for many years. However, due to the lack of compact and bright sources there has long been a gap in commercial applications using THz radiation. In recent years, the advent of new optoelectronic generation and detection schemes as well as new technologies for integrated mm-wave components has led to first commercial applications. Still, the question how to measure power, power density, waveforms and material parameters accurately at sub-mm wavelengths remains unsolved. Today, the National Metrology Institutes are faced with a number of metrological challenges that need to be solved in order to help this new technology become marketable.