In order to understand the complicated chemical and physical processes that occur during the deposition of hard face coatings such as diamond, experiments that are remote, nonintrusive and sensitive to critical chemical species have been performed. Coherent anti-Stokes Raman spectroscopy (CARS) has been used to measure temperature and detect species such as methane and acetylene under low pressure, CVD environments. Results of these experiments for both an rf PACVD and heated-filament apparatus are described. In addition, these results and literature studies are interpreted using modeling (kinetic and equilibrium) calculations. Intepretations of the experimental results confirm the importance of high concentrations of hydrogen atoms, suggest that (hydrocarbon) radical species play a negligible role, and support proposals that in the presence of reactive hydrogen atoms virtually any hydrocarbon (or hydrocarbon oxygenate) can lead to diamond growth. The results in other laboratories on diamond deposition in acetylene/oxygen flames strongly support the first of these interpretations. In order to understand the competive process of soot/amorphous carbon formation, the equilibrium analysis of Stein and Fahr has been extended to low pressure, diamond forming conditions. This study indicates that a thermodynamic barrier exists to the growth of polyaromatic hydrocarbons at temperatures above 1300 to 1400K, pressures of 25 torr and hydrogen/acetylene ratios of 200.