In the microwave induced plasma deposition of diamond from methane-hydrogen-oxygen mixtures, the variables available for controlling the microstructure of the resulting films are the plasma composition and density, the substrate surface properties, and the temperature. It has been demonstrated that the competition between nucleation site formation and the rate of reactive plasma etching is the critical feature in the development of the film microstructure on silicon substrates. On diamond substrates, the favorable lattice match dominates and homo-epitaxial films are formed over a wide temperature range. Raman scattering studies also demonstrate that the reactive etching of the diamond surface by oxygen-containing species is critical to the removal of non-diamond carbon species over the temperature range 450–1050 °C.