In this thesis we studied the toughening effects of graphene derivatives, which have drawn much attention recently due to their high aspect ratios and outstanding mechanical properties. Graphene-based toughener can toughen resin at extremely low loading levels, which means it is economically viable for price-driven thermosetting resins market. To understand the toughening effect of graphene derivatives in resins, several GO surface modifications were developed to help disperse GO into the resins. The best performing modified GO (mGO) investigated in this work can be homogeneously dispersed into a resin with merely mechanic mixing. To simplify the materials handling and further improve the toughener dispersion, a styrene masterbatch route was developed to avoid the freeze-drying step in the mGO synthesis. The toughening effect of pristine and modified graphene oxide was tested in both unsaturated polyester and vinyl ester resins. The result indicated that GO and its derivatives can toughen UP and VE resins at a loading lower than 0.04 wt.%. Although, these tougheners are highly efficient in terms of required loading, we found that the toughness improvement obtained by adding mGO is insensitive to changes in particle-matrix interfacial strength and toughener loading. To understand this behavior, we studied the inorganic filler interference to mGO toughening, and also how the mGO toughening effect is affected by the physical dimensions of GO size and mGO aggregate size. Sophisticated data analysis involving computerized particle analysis were carried out to characterize the size differences between samples. The results show that the toughening effect of mGO is identical to that of other inorganic fillers, and this toughening effect is independent of filler mechanical properties. Finally, the toughening performance of mGO was tested in glass fiber reinforced composites, which is the target product for UP and VE resins. Both the interlaminar fracture toughness test and Izod impact test showed no improvement in composite toughness after adding mGO. A detailed fractography analysis of failed composite samples indicate that the failure happens between the resin and the glass fiber, which means increasing the fracture toughness of the resin matrix will not likely show any effect on the composite fracture toughness.