In this book, a systematic study of laser irradiation effects in polymers and crystals is presented. Several microstructures, craters, diffraction gratings, waveguides and Y couplers were fabricated in polymers and Lithium Niobate crystal using fs laser direct writing technique. Four different polymers PMMA, PDMS, PS, and PVA were investigated in the study. The formation of defects which are responsible for the observation of emission in polymers is explained in the context of formation of myriad optical centers. The change in emission peak with excitation wavelength is observed in polymers and explained through "Red edge effect". The observation of ESR signal in fs laser irradiated polymers is interpreted due to the formation of paramagnetic centers. Broadening and suppression of Raman modes in case of irradiated polymers indicated the presence of defects in polymers. Fs laser irradiation on Lithium Niobate crystal has not shown formation any optical or paramagnetic centers. Also the synthesis of silver nanoparticles through laser irradiation (ns,ps, and fs) in PVA matrix is presented in this study. Linear and nonlinear optical properties of silver nanoparticles are reported.