Nanocomposites based on polymer matrices have been studied extensively. In the majority of these studies, the focus has been the enhancements in properties through the addition of relatively small quantities of nanoparticles. In the chapter, we centre our attention on the effects, if any, of the nanoparticles on the behaviour of the polymer matrix and in particular the nucleation and growth of crystal phases in the case of crystallizable polymers. We first consider the influence of macroscopic particles on this behaviour by examining the influence of polymer fibres on the matrix behaviour. We then consider the situation in polydisperse melts where extended objects can be formed from the high molecular weight fraction in the melt, and then finally we review work reported in this field in the context of nanoscale fillers.