In this dissertation work we have developed an ultracompact optically-detected terahertz resonance (ODTR) spectroscopy system, using quantum cascade lasers (QCLs). This system can be used for investigating various terahertz (THz) resonances in semiconductors. We use a THz QCL, of an appropriate frequency, operating in the close vicinity of the sample. Due to the small size of the QCLs, we can mount them on the sample holder of the magnet, which allows focusing of the THz beam onto the sample with minimal loss. This precludes the need of any external source of radiation. We use a single fiber to send the pump laser beam to the sample and collect the PL signal. The use of the fiber makes our system versatile enough to be used in a variety of situations, especially inside our magnet. We have used this setup to obtain some preliminary ODTR data on an InGaAs/AlGaAs multiple-quantum well sample.