Continuous-wave (cw) optical parametric oscillators (OPOs) are working horses for spectroscopy for near and mid infrared wavelengths. However, in the terahertz frequency range (0.1 to 10 THz), the pump threshold for oscillation is larger than 100 W due to high absorptions and thus exceeds the power of standard cw single-frequency pump sources. In this thesis, the first cw OPO capable of generating terahertz radiation is demonstrated. To overcome the threshold, the signal wave of a primary infrared process is resonantly enhanced to serve as a pump wave for a cascaded parametric process with...