The availability of THz frequency comb sources is a key enabler for spectroscopy and chemical sensing using characteristic molecular absorption lines. In recent year, large progress has been made with THz quantum cascade laser combs. We show in these contribution novel approaches to achieve THz frequency combs. Ring resonators are an interesting alternative cavity solution to the commonly used ridge type waveguide for THz Quantum Cascade lasers. They either support a standing wave pattern showing spatial hole burning if there are defects implemented or a traveling mode in a defect-free cavity. We have studied a ring-shaped THz Quantum Cascade lasers emitting between 3.2 and 4.1 THz operating in four different emission regimes. The presence of defects in the cavities force the THz Quantum Cascade laser into a standing wave pattern. The measurements show a complex behavior highlighting the effect of strong confinement and the optical nonlinearities leading to the generation of a harmonic state, as well as to a fundamental comb, exhibiting over 30 equidistant modes and covering a bandwidth of 622 GHz. In a second approach, we use a novel combination of optical fibre communications technology and opto-electronic frequency conversion. We use a near-infrared laser source which is modulated by an electro-optic modulator. The generated side-band spectrum is then combined with a second near-infrared laser and sent to a photoconductive antenna where the difference frequencies are converted into a THz comb. With this approach, we demonstrate a method for creating a terahertz frequency comb that is flexible in terms of setting the comb’s central frequency and the number and spacing of the comb lines. The combs are analyzed by a Fourier transform spectrometer (FTS) and cover a range of 1.1-3.5 THz.