Leptogenesis is an appealing scenario which can explain why there is more matter than antimatter in our Universe. It only requires a minimal extension of the Standard Model by at least two heavy right-handed neutrinos, which is well motivated by the observation of neutrino oscillations and by the see-saw mechanism. After a general introduction to leptogenesis I discuss its theoretical description in terms of rate equations. I show that the coefficients in these equations can be obtained by computing correlation functions in finite temperature field theory.