Graphene is the ultimately thin crystal, a purely two dimensional plane of carbon atoms arranged on a honeycomb lattice. This material has stimulated a worldwide research activity because electrons in graphene have a linear electronic dispersion and hence, behave as massless particles, similar to those encountered in high energy physics. We use optical techniques (infrared transmission/reflectivity and Raman scattering) combined with high magnetic fields to study the effects of electron-electron and of electron-phonon interactions in mono and multilayer graphene, with different stackings.