THE FIRE HAZARDS INHERENT IN OXYGEN SYSTEMS make materials selection a crucial step in designing and maintaining a safe system. To ensure the safety of any oxygen system, the system designer must have an understanding of the numerous factors relating to the selection of suitable materials for oxygen service, including material properties related to the design and operating conditions, compatibility with the operating environment, ignition and combustion behavior, property changes that occur at cryogenic temperatures, and ease of fabrication, assembly, and cleaning. The focus of this chapter is materials selection related to flammability, ignition, and combustion. Information on other areas of materials selection, such as mechanical and thermal properties of engineering materials, is located in Appendix B.