The advent of optical frequency combs revolutionized many research fields from metrology to high precision spectroscopy. It was recently demonstrated that broadband quantum cascade lasers can operate as frequency combs. As such, they operate under direct electrical pumping at both mid-infrared and terahertz frequencies, reaching powers in the watt range with multi-terahertz bandwidths. As their key application field, they unlock the advantages in speed and accuracy of the dual-comb spectroscopy technique in a frequency range where molecules have their fundamental vibrational and rotational bands. In this Chapter we review the design and basic functioning principles of these devices, the characterization of their coherence properties as well as few example applications.