Brittle matrix composites, including carbon-carbon (C-C) and ceramic matrix, offer a new dimension in the area of high-temperature structural materials. Fiber-matrix interactions determine the mechanism of the load transfer between the fiber and matrix and resulting mechanical properties. Composites studied in this work include a C-C composite densified with a chemical vapor infiltration (CVI) pyrolytic carbon, silicon carbide fiber-silicon carbide matrix composite, and carbon fiber-silicon carbide matrix composites densified by the CVI technique. The type of the interfacial carbon in C-C composites was found to control their mechanical properties. The presence of the compressive stress exerted by the matrix on the carbon fibers was attributed to an increase in flexural strength. The transverse matrix cracking in C/SiC composites was believed to cause a lowering in the flexural strength value. Brittle fracture behavior of SiC/SiC composites was correlated with the presence of an amorphous silica layer at the fiber-matrix interface.