Research into terahertz technology has drawn increasing attention in recent years, giving rise to a number of potential applications which have in turn promoted advances in terahertz sources. This chapter outlines such applications and sources and the key steps in the development of the Quantum Cascade Laser (QCL). This provides a route for the discussion of various laser designs, the introduction of the Si/SiGe based QCL and the motivation for the pursuit of such a device.