Recent developments in terahertz quantum cascade lasers are reviewed. Structures operating from a wavelength of lambda = 66 microm down to lambda = 87 microm are demonstrated. These devices used either a three-quantum-well chirped-superlattice active region or an active region based on a bound-to-continuum transition. The comparison between structures grown in a waveguide based on a single interface plasmon and a buried contact and (non-lasing) structures using a double plasmon waveguide demonstrates the importance of waveguide design on the operation of such devices. Continuous-wave operation up to a maximum temperature of 55 K with up to 15 mW output power at 10 K was demonstrated.