Wave Function Renormalization at Finite Temperature

We present a derivation of the medium dependent wave function renormalization for a spinor eld in presence of a thermal bath. We show that, as already pointed out in literature, projector operators are not multiplicatively renormalized and the e ect involves a non trivial spinor dependence, which disappears in the zero temperature covariant limit. The results, which di er from what already found in literature, are then applied to the decay of a massive scalar boson into two fermions and to the {decay and crossed related processes relevant for primordial nucleosynthesis.