We extend the results of Denisov-Rakhmanov, Szegő-Shohat-Nevai, and Killip-Simon from asymptotically constant orthogonal polynomials on the real line (OPRL) and unit circle (OPUC) to asymptotically periodic OPRL and OPUC. The key tool is a characterization of the isospectral torus that is well adapted to the study of perturbations.Underlying the association of measures and recursion coefficients are matrix representations. For OPRL, we take the matrix for multiplication by x in the