Since 1880, when first applied in the incandescent lamp industry, chemical vapor deposition (CVD) has been employed in a diverse group of technologies. At present, CVD plays vital roles in microelectronic, wear- and radiation-resistant coatings, fiberoptics, and the purification and fabrication of exotic materials, from ultra-low expansion glasses to high-purity refractory metals. In the CVD process, a precisely mixed stream of gases is injected into a cavity where it encounters a heated substrate, which provides the thermal activation energy that initiates a chemical reaction in the vapor. This reaction leaves behind a solid film on the substrate surface, while the other reaction products are entrained in the gas stream and exhausted from the cavity. The reaction normally takes place below or at atmospheric pressure, and the substrate can be heated by optical, infrared, or radio frequency radiation. CVD has four major advantages over most other thin film deposition techniques. First, the process allows tight control over gas stream flowrate and composition, which leads to predictable and repeatable film composition and to graded structures, if desired. Second, the thermal activation of the reaction establishes thermal equilibrium at the site of film deposition, producing tight, highly coordinated structures. Third, the throwing power of CVD is excellent. Last, the technique is especially well suited to the deposition of refractory materials difficult by other processes. Therefore, the application of this technology to optical thin films promises to be fruitful; the important steps have been taken in a number of areas.