Absence of CP Violation in the Strong Interaction: Vacuum thwarts Axion

QCD admits a contribution to the action, the $\theta$ term, which potentially gives rise to nontrivial phases and violates CP. This is essentially a question of how the vacuum reacts to the $\theta$ term. In this talk I will address the problem using new developments on the lattice. The overall solution is contrasted with the axion `solution’.