Four types of materials were evaluated in this study : neat epoxy resins of differing initial molecular mass, and their rubbery particle-filled, rigid particle-filled, and hybrid particle-filled counterparts. Both rubbery and rigid particle-filled epoxies exhibited fracture energies, G(Ic), greater than the neat resins. Cured resins prepared with an optimum loading of both rubbery and rigid particles gave G(Ic) values greater than those from rubbery or rigid particles alone (approximately sixteen-fold increase over the neat resins). Rigid particle composites were toughened by means of a crack front impedance mechanism. In hybrid particle composites, rigid particles enhanced localized plastic deformation ahead of the crack front.