The fatigue crack propagation (FCP) resistance of epoxy-based composites containing various concentrations of solid glass spheres (SGS) and/or reactive liquid rubber (CTBN) was examined. The FCP results show that the simultaneous use of rubber and solid glass spheres (hybrid composites) results in synergistic improvement in FCP resistance of composites through the entire crack growth regime. The nature of synergistic interactions was elucidated by careful examination of the fatigue fracture surfaces and the subfatigue fracture surfaces of fatigue samples. It was shown that when rubber particles cavitate in the vicinity of the glass spheres, regardless of the nature of the interface, glass particle debonding from the matrix is suppressed due to a change in the crack tip localized stress state. This, in turn, results in improved pinning/bridging efficiency of the glass spheres. Furthermore, it was shown that crack tip plastic zone-rubber particle interactions induce a transition in FCP behavior of rubber-modified epoxies. Consequently, crack tip shielding mechanisms become active when the size of the plastic zone at the crack tip becomes large compared to the size of the rubber particles.