PolYMers subjected to ion beam or gamma irradiation undergo structural modifications, which lead to an increase in refractive index for some materials, such as poly(diethylene glycol bis(allyl carbonate)) (CR39) and polycarbonate. The refractive index variation achieved with ion beam irradiation is sufficient to produce optical waveguides in CR39. The waveguide losses have been systematically studied in order to apply this technique to integrated optics (microstructure fabrication). A fundamental study using various spectroscopic methods (i.r. and u.v.-visible) has also been carried out, with the aim of gaining a better understanding of the structural modifications of polymeric materials caused by irradiation.