Flaw-tolerant ceramic matrix composites typically possess an interphase between the reinforcing fibers and the matrix that allows debonding for desirable mechanical properties. However, commonly used interphase materials, such as carbon and boron nitride, are susceptible to oxidation at elevated temperatures. In general, oxidation of interphase materials produces gases and voids between the fibers and the matrix leading to an increase in the compliance of bridging fibers and a subsequent decrease in the toughening increment due to fiber bridging. On the other hand, silicon carbide fibers and matrices oxidize to form solid reaction products (oxides), leading to increased bonding between fibers and the matrix. A competition between interphase oxidation and the formation of solid oxide phases determines the composite failure mechanism under a given set of environmental conditions. The mechanisms by which oxidation degrade the mechanical properties of SiCf/SiCm will be discussed. Techniques for identifying failure mechanisms and approaches to modeling component lifetimes will be presented.