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A diglycidyl ether of bisphenol A (DGEBA) epoxy resin was modified by incorporation of varying concentrations of hollow glass spheres (HGS) and/or reactive liquid rubber (CTBN). The fatigue crack propagation (FCP) behavior and mechanisms of such materials were studied in detail. A synergistic phenomenon reported for static fracture toughness was also observed in the FCP resistance of such composites. Optical microscopy studies revealed that the interactions between the stress fields of the crack-tip process zone and HGS cause plastic-zone branching, which in turn gave rise to synergistic toughening. It is also shown that process zone second phase particle interactions cause a transition in FCP behavior of rubbermodified epoxy polymers, similar to that observed in metal alloys. Consequently, the process zone toughening mechanisms are active only above a certain stress intensity range. Conversely, FCP resistance of both modified and unmodified epoxies were the same below the transition.