We have developed a coherent time-domain THz magneto-spectroscopy system. It can be used for measuring the refractive index, complex conductivities, and cyclotron resonance of different semiconductor structures as well as for investigating fundamental physical phenomena. As a first application, we have carried out time-domain cyclotron resonance studies of an ultrahigh-mobility two-dimensional electron gas at low temperatures. We observed coherent cyclotron resonance oscillations that persist as long as ∼ 50 ps. We show that the basic physics of these oscillations can be described as the free induction decay of a coherent superposition between the lowest unfilled Landau level and the highest filled Landau level prepared by an incident THz pulse. Using the 0 Tesla data as a reference, we successfully extracted the real and imaginary conductivities for different magnetic fields in the frequency domain. Finally, I will discuss a few future experiments that will be performed with this novel system.