Ultra-short laser pulses provide unique possibilities not only for precise laser treatment of optical materials but also for the fabrication of sub-wavelength structures both on the surface and in the volume of transparent materials. These structures have numerous applications in optics, photonics, telecommunications, medicine, and many other fields. The mechanisms involved are based on material ionization, light propagation, scattering, and absorption. In a multipulse regime, femtosecond laser pulses can induce a set of distributed defects and/or inhomogeneities leading to an additional local field enhancement or to several localized hot spots, accompanied by an additional electronic excitation, nanopore formation, densification, etc. These processes are investigated by using the developed multi-scale modeling that allows explanations of several fascinating effects, such as volume nanogratings and other periodic structure formation. In this talk, modeling details and several simulation results will be presented providing insights into the effects induced by multi-pulse ultrashort laser interactions with initially transparent solid materials. It will be shown how based on the presented analysis, it is possible not only to determine optimal laser parameters but also to predict structure properties.