The quantum-cascade lasers (QCL), first demonstrated in 1994, has since been developed into a mature laser emitting within nearly the entire spectrum from 2.6 to 250 μm, particular within the mid-infrared part of the spectrum from 3 to 12 μm for applications in gas sensing for security, environmental and medical uses, as well as for defense-related IR countermeasures. The QCL heterostructure is generally based on the InGaAs/InAlAs system lattice-matched to InP or on its strain-compensated extension to maximize the conduction band discontinuity between well and barrier material. A refinement is the use of mixed-height barriers to engineer the interface scattering of the different levels involved in the lasing process. This design strategy appears to be universally applicable, across the entire range of QCL emission wavelengths. By using low barriers where the upper laser state has its maximum probability and high barriers where the lower laser state has its maximal probability in strain-compensated designs for short wavelength emission, the lifetime of the upper laser state can be increased, while decreasing the lifetime of the lower laser state. First realizations of this design result in Jth = 1.7kA/cm2 at 300 K, slope efficiency η = 1.4 W/A, T0 = 175 K, and T1 = 550 K. Further increases in efficiency can be achieved through designs in which parasitic states near the upper laser level are separated from it, either energetically or oscillator strength. These states may be associated with other k values, or with higher-lying subbands.