New issues in quark-lepton symmetry

Two models incorporating different forms of spontaneously broken quark-lepton symmetry are discussed. Both models are constructed so that quark-lepton symmetry can be broken at as low an energy scale as phenomenology allows, thus maximising their testability. The first model uses the Pati-Salam gauge group SU(4)$\otimes$SU(2)$_L\otimes$SU(2)$_R$. The main analysis presented here concerns large threshold corrections to the $m_b = m_{\tau}$ tree-level mass relation. The second model is a new and simplified theory using discrete quark-lepton symmetry which has a very economical Higgs sector and elegantly avoids a potential quark-lepton mass relation problem.