A study is presented of increasing the toughness of Cyclic Olefin Copolymer (COC) while maintaining its optical transparency. The COC consists of a random copolymer of ethylene and norbornene, and the impact modifiers consist of thermoplastic elastomers. It is shown that several requirements must be satisfied, namely: for toughening there is an optimum finite particle size, but in order to minimize light scattering the particles have to be as small as possible. In addition, the refractive index of the elastomer must be closely matched to COC over the visible wavelength range and use temperature. Also for toughening, there are additional requirements of the adhesion of the elastomer to COC and high molecular weight. It is found that styrene butadiene styrene (SBS) is the most effective elastomer in toughening COC while maintaining optical transparency. It is also found that the addition of an index matched styrene-ethylene-butylene-styrene (SEBS) copolymer as a compatibilizer to the SBS elastomer is beneficial in increasing the toughness and lowering the optical haze. Finally, light scattering calculations are presented based on the Rayleigh Debye model to calculate the optical haze and transmission of these blends. These calculations take into account the refractive Index of COC and the elastomer, the particle size distribution and volume fraction of the elastomer. It is shown that there is reasonable agreement between calculation and experimental results. It is possible to increase the toughness of COC to greater than 50 J/m (Notched Izod) while keeping the optical haze to below 5% with an elastomer loading of 5% (w/w). We also identify opaque blends of COC with a toughness of greater than 500 J/m with an elastomer loading of 20% (w/w).