Terahertz quantum cascade laser sources based on intra-cavity difference frequency generation are currently the only electrically-pumped monolithic semiconductor light sources providing broadly-tunable terahertz output at frequencies up to 6 THz at room temperature. Relying on the active regions with the giant second-order nonlinear susceptibility and the Cherenkov phase matching scheme, these devices demonstrated drastic improvements in performance in the past several years and can now produce narrow-linewidth single-mode terahertz emission that is tunable from below 1 THz to almost 6 THz with power output sufficient for imaging and spectroscopic applications. This chapter provides a comprehensive overview of this device technology