Quantum cascade lasers (QCLs) emitting in the 4-10 micron wavelength range are treated with emphasis on key issues not covered in previous books on QCLs. The foremost issue discussed: what does it take to achieve continuous-wave (CW) operation to multi-watt powers in a highly efficient manner, is of interest to a wide range of applications. A comprehensive review of the temperature dependence of the electro-optical characteristics of QCLs is presented by including elastic scattering and carrier-leakage triggered by elastic and inelastic scattering, thus accounting for all mechanisms behind the device internal efficiency. Maximizing the CW wall-plug efficiency via conduction-band and elastic-scattering engineering, and photon-induced carrier transport is treated in detail. Then coherent-power scaling is discussed for both one- and two-dimensional (2-D) structures with emphasis on the optimal solution: high-index-contrast (HC) photonic-crystal (PC) lasers. Grating-coupled surface-emitting lasers are also treated with emphasis on those needed for 2-D HC-PC lasers; that is, devices most likely to operate in diffraction-limited, single-lobe beam pattern to multi-watt CW output powers