This chapter provides an overview of a class terahertz quantum cascade lasers based upon amplifying electromagnetic metasurfaces. The metasurface comprises two-dimensional arrays of sub-wavelength surface radiating antenna elements, in which the antennas are loaded with the quantum cascade laser gain material. Several types devices are described: (a) vertical-external-cavity surface-emitting-lasers (VECSELs) in which the amplifying metasurface is paired with external optics to form a laser cavity; (b) monolithic metasurface lasers in which the metasurface array self-oscillates in a coherent supermode; and (c) metasurfaces which operate below threshold as free-space terahertz amplifiers. The metasurface approach allows the realization of large-area radiating apertures while preserving the sub-wavelength sized of the individual metallic waveguide antenna elements. This has resulted in significantly improved performance and functionality in many categories, including lasers with high-quality beam patterns, high-efficiency lasers with scalable output powers, broadband spectral tunability of single-mode emission, and free-space amplification of terahertz beams.