This appendix is dedicated to the body of results about structured ring spectra which are relevant to the String–orientation of elliptic spectra. We include four main topics: a discussion of the stable homotopy theoretic analog of character theory; the compatibility of the results of the preceding chapter on orientations in the homotopy category with power operations for Lubin–Tate spectra; the sheaf of ring spectra enriching the moduli stack of elliptic curves and the spectrum of topological modular forms; and orientations of the spectrum of topological modular forms by maps of structured rings.