Longitudinal tensile and flexural fatigue behavior of several short and long glass reinforced thermoplastic materials were compared through stress versus cycles to failure diagrams, fatigue crack propagation theories, and microscopic crack observations. Differences in fatigue behavior between polypropylene, nylon, and polyphthalamide long fiber composites were primarily due to resin crack propagation criteria coupled with interfacial debonding behavior. The interfacial strength of polypropylene glass reinforced composites is particularly important due to inherent glass/polypropylene incompatibility issues. Although the choice of a good chemical coupling agent improves the interfacial bond strength of glass reinforced polypropylene, crack initiation and propagation in the interface was still much more evident in chemically coupled polypropylene composites versus the other thermoplastic composites. Flexural fatigue results indicated that the polyphthalamide long glass reinforced material exhibited the best fatigue behavior followed by nylon and polypropylene. This trend follows the expected mechanical behavior differences between the base resins. Longitudinal tensile and flexural fatigue behavior of long versus short fiber composites were also examined. In general, long fiber fatigue benefits were not observed in the tensile fatigue mode since fibers were not an integral part of the crack propagation mechanism. Fatigue benefits only arose when fiber breakage, fiber pullout, and crack opening displacement mechanisms strongly influenced the crack growth rate. These phenomena were observed in flexural fatigue. The slower crack propagation rates that occurred in the long fiber materials led to superior flexural fatigue performance versus their respective short fiber counterparts. This is important since the long fiber reinforced thermoplastics are seeking applications in important automotive applications such as battery trays, transmission covers, side view mirror brackets, starter gears, and other potential areas.