Since the early 90’s it was understood that in order for the Peccei Quinn (PQ) mechanism to work the global U(1)_PQ had to be realized to an extremely high level of accuracy. In fact, even a tiny explicit breaking of the global PQ symmetry (e.g. via Planck-induced effective operators) would sizably contribute to the axion vacuum energy density and prevent the θ angle to relax to a value below the experimental upper bound |θ| < 10^-10. In this contribution, I will reproduce the standard argument and review possible mechanisms in order to protect the axion.
