Heteroepitaxy is an important area of materials research because of its applicability to a variety of technological applications. One area presently attracting a great deal of research is heteroepitaxy of electronic and optoelectronic materials. Here the word heteroepitaxy is emphasized to distinguish it from the much more common epitaxial growth of essentially the same material where only variations in the concentration of dopants is allowed. When materials scientists and engineers from the electronics industry refer to heteroepitaxy, they mean the oriented-crystal growth of some interesting (at least, to them!) material on a single-crystal substrate of some other material. By using heteroepitaxy, it is possible to tailor the electronic or optical properties in a way that is just not possible with doped, epitaxial growth of the same material. There are some rare examples of pairs of materials that have the same crystal structure, are very closely lattice-matched, and are dissimilar enough in their optical and/or electronic properties to make heteroepitaxy interesting. One such couple, AlxGa1−xAs/GaAs (lattice mismatch of 0.1% when x = 1), has been extensively exploited over the last 30 years.