  Since the fifties, isospin relations have been used in particle physics to
understand the properties of multihadrons final states. In the case of the tau
lepton, they allow to relate the partial widths of the decay modes to the cross
sections of e+e- annihilations. A pedagogical introduction to the construction
of isospin states for meson systems and an updated review of the use of isospin
relations in the study of the tau lepton are presented.
