GaInAs/AlInAs/InP quantum cascade lasers have established themselves as reliable laser sources in the mid-infrared region (3.8-10) μm, where they operate at room-temperature in continuous-wave with Watt-level output powers. However, wavelengths above this wavelength region are difficult to generate. At long wavelengths, devices suffer from increased free-carrier absorption and poor population inversion due to the short upper laser state lifetime, thus limiting their operation to cryogenic temperatures. An alternative way to generate new frequencies is the by means of nonlinear frequency mixing. For long-wavelengths, the process of difference frequency mixing is of particular interest, as it is possible to utilize the good performance of the mid-infrared QCLs, acting as pump sources, together with the giant nonlinear properties that can be realized in the intersubband transitions of the quantum wells. Moreover, the giant nonlinearity can be monolithically integrated with the pump sources, leading to a compact, electrically pumped room-temperature semiconductor laser source, emitting at terahertz frequencies. In our work, we present several different concepts of monolithic nonlinear quantum cascade laser sources, designed to emit in the THz range: devices with passive giant nonlinearities, active nonlinearities and, finally, devices with active nonlinearities, combined with novel THz waveguiding techniques. We will demonstrate how application of novel THz waveguiding techniques avoids the efficiency suppression the large free-carrier absorption at THz frequencies in the doped semiconductor layers enabling room-temperature operation up to 1.2 THz.