The Terahertz (THz) region of the electromagnetic spectrum is usually defined as ranging from 1 THz (300 micrometers in wavelength) to 10 THz (30 micrometers in wavelength). In this dissertation we present the complete realization of semiconductor-heterostructure lasers emitting in the THz and employing the quantum cascade technology. The quantum cascade laser is a coherent light source based on optical transitions between discrete electronic states arising from size confinement in semiconductor quantum wells. The quantum mechanical design of the heterostructure-based gain medium is discussed in detail, together with the different optical resonators employed, the fabrication process and the measurement techniques. Different active region designs are presented and discussed together with extensive experimental data. The effects of a further confinement of the electrons induced by the application of intense magnetic fields perpendicular to the plane of the layers are presented. An in-depth experimental study on different active regions demonstrates the radical change in the physics of these lasing systems introduced by the magnetic confinement.