In August/September 2000, a group of 80 physicists from 53 laboratories in 15 countries met in Erice, Italy, to participate in the 38th Course of the International School of Subnuclear Physics. This book constitutes the proceedings of that meeting. It focuses on the theoretical investigation of several basic unity issues, including: (1) the understanding of gauge theories in both their continuum and lattice versions; (2) the possible existence and relevance of large extra dimensions together with the resultant lowering of the Planck/string scale to the TeV range; (3) the origin and structure of flavour mixing in the quark and lepton (neutrino) sectors.