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.