In this chapter, the realisation of semiconductor lasers which operate continuously at room temperature with reasonably low currents is not possible without the use of heterostructures. In a normal double heterostructure (DH) laser the hetero structures provide the refractive index step which is necessary to localise the light in a waveguide, and the difference in band gap which are necessary to confine both electrons and holes to the same region of the structure. The individual discontinuities in the conduction and valence bands are used further in a quantum well laser to produce beneficial 2-dimensional effects in the electronic structure of the active region material and to modify the operating wavelength by quantum size effects. While all these intrinsic aspects of heterostructures are of great benefit to the laser, it is important to avoid deleterious effects due to the extrinsic properties of the structure, such as poor quality interfaces between the heterostructure components and unwanted non radiative leakage currents, which can increase the total operating current.