This paper traces the historical development of high temperature resistant rigid-rod polymers. Synthesis, fiber processing, structure, properties, and applications of poly(p-phenylene benzobisoxazole) (PBO) fibers have been discussed. After nearly 20 years of development in the United States and Japan, PBO fiber was commercialized with the trade name Zylon (R) in 1998. Properties of this fiber have been compared with the properties of poly(ethylene terephthalate) (PET), thermotropic polyester (Vectran (R)), extended chain polyethylene (Spectra (R)), p-aramid (Kevlar (R)), m-aramid (Nomex (R)), aramid copolymer (Technora (R)), polyimide (PBI), steel, and the experimental high compressive strength rigid-rod polymeric fiber (PIPD, M5). PBO is currently the highest tensile modulus, highest tensile strength, and most thermally stable commercial polymeric fiber. However, PBO has low axial compressive strength and poor resistance to ultraviolet and visible radiation. The fiber also looses tensile strength in hot and humid environment. In the coming decades, further improvements in tensile strength (10-20 GPa range), compressive strength, and radiation resistance are expected in polymeric fibers. Incorporation of carbon nanotubes is expected to result in the development of next generation high performance polymeric fibers. (c) 2005 Wiley Periodicals, Inc.