Review of constraints on fermion mixing

The constraints on the mixing angles of the standard fermions with new heavy particles that can appear in many extensions of the electroweak theory are reviewed. Some emphasis is put in distinguishing the effects of a mixing with new states which transform in a non-canonical way with respect to $SU(2)\times U(1)$ (left-handed singlets or right-handed doublets), from the effects of a mixing with new states with standard quantum number assignments. Constraints from flavor changing neutral current processes, as well as from flavor diagonal and charged current experimental data are considered. New limits for lepton flavor violating mixings are presented. By using the most recent results on the $\tau$ mass, lifetime and branching ratios, updated limits on the mixing parameters of the $\tau$ neutrino are derived. These limits are improved up to a factor of 3, and no signals of deviation from the standard model predictions are found. Limits on the mass of a $Z_1$ gauge boson from E$_6$, from the absence of muon number violating processes are briefly discussed as well.