  We discuss the implications of large neutrino mixings for grand unified
theories based on the seesaw mechanism. In SU(5) GUTs large mixings can be
accomodated by means of $U(1)_F$ flavour symmetries. In these models the heavy
Majorana neutrinos are essentially decoupled from low energy neutrino physics.
On the contrary in SO(10) GUTs large neutrino mixings severely constrain the
mass spectrum of the heavy Majorana neutrinos. This leads to predictions for a
variety of observables in neutrino physics as well as for the cosmological
baryon asymmetry.
