The fatigue crack propagation behavior of glass-filled epoxy composites with three different types of reinforcements was characterized. Moreover, the underlying mechanisms operating in fatigue were identified using da/dN vs. ΔK curves according to the model proposed by other investigators. The effect of moisture was studied. In particular, the role of adhesion promoters in fatigue crack ... [Show full abstract] propagation behavior was determined by treating the glass reinforcements with silane-based adhesion promoters. The fatigue crack propagation behavior with moisture exposure was strongly dependent on the surface treatment of the reinforcements and poor matrix–particle adhesion resulted in better fatigue crack propagation resistance. An investigation revealed that the specific toughening mechanism contributing to fatigue crack propagation behavior depended on the type of reinforcement and the surface treatment. Glass spheres treated with n-butyltrimethoxysilane exhibited crack tip shielding mechanisms involving shear yielding whereas short glass fiber treated with aminopropyltrimethoxysilane exhibited contact shielding mechanisms involving fiber bridging and wedging due to asperities generated between fracture surfaces. Interestingly, the type of adhesion promoters had a significant influence on the type of toughening mechanism observed.