QCD Vacuum and Axions: What's Happening?
  A deeper understanding of the vacuum structure in QCD invites one to rethink certain aspects of the axion physics. The recent advances are mostly due to developments in supersymmetric gauge theories and the brane theory, in which QCD can be embedded. They include, but are not limited to, the studies of metastable vacua in multicolor gluodynamics, and the domain walls. We briefly review basics of the axion physics and then present a modern perspective on a rich interplay between the QCD vacuum structure and axion physics. 
Gregory Gabadadze, M. Shifman