The high-strength, high-modulus Kevlar 49 fiber is widely used today because of its superior properties. In this paper, we discuss the fundamental physicochemical nature of the commercial fiber. It is an extended chain polymer, poly(p-phenylene terephthalamide), which is highly crystalline. Extensive analysis shows that the material composition is quite consistent from lot to lot and is low in impurities. The fiber absorbs water reversibly; the extent of absorption is related to the ash content. The well-known interaction of the fiber with ultraviolet light is illustrated with spectra, and the thermal stability of the fiber is demonstrated with various thermal analysis techniques.