The terahertz frequency range falls between 0.3-10 THz and represents an underutilized part of the electromagnetic spectrum. In particular, light in the 2-10 THz range is difficult to generate, as parasitics negatively impact high-frequency electronics and conventional laser sources do not operate in this range. Our research in this area uses a combination of terahertz quantum cascade lasers and terahertz time-domain spectroscopy to study the physics of various systems, including quantum cascade lasers and topological insulators.