TASI Lectures on The Strong CP Problem
  These lectures discuss the $\theta$ parameter of QCD. After an introduction to anomalies in four and two dimensions, the parameter is introduced. That such topological parameters can have physical effects is illustrated with two dimensional models, and then explained in QCD using instantons and current algebra. Possible solutions including axions, a massless up quark, and spontaneous CP violation are discussed. 
Michael Dine