  Any would-be Peccei-Quinn (PQ) symmetry is vulnerable to various types of
explicit breaking. It has long been recognized that these can disrupt the axion
solution to the strong CP problem. There have also been suggestions that, under
certain circumstances, these can lead to a surprisingly large mass for the
axion. Two types of corrections to this computation have been widely
considered: higher dimension symmetry breaking operators, in theories where
there is a complex field responsible for the spontaneous breaking of the PQ
symmetry, and small instantons. Motivated by situations where small instantons
dominate the $\theta$ potential of non-abelian gauge theories, we formulate the
question of axion quality in terms of constraints on a Wilsonian effective
action at scales somewhat above the scale of QCD. In this language, the
standard axion mass computation assumes a nearly exact PQ symmetry in this
action. The higher dimension operators and/or small instantons represent
symmetry breaking terms in the Wilsonian action. This picture permits a uniform
treatment of the problem of {\it axion quality}. If one assumes order one CP
violation at high energies, then solving the strong CP problem constrains the
axion potential terms in this Wilsonian action; in particular, the axion mass
must be extremely close to its "standard" value.
