Solid state physics in its early days in Arnold Sommerfeld's and Werner Heisenberg's groups, where Peierls was a student in the late 1920s. Sommerfeld's group compared to Heisenberg's group; interaction between experimentalists and theoreticans in Germany, in England and in the U.S.; Peierls' thesis work on heat conductivity in non-metallic crystals (suggested by Wolfgang Pauli), done in Zurich in 1920 and defended at Universitt̃ Leipzig; the application of the Heilter-London approach in other works (especially Felix Bloch, Arnold Sommerfeld); attempt to state the first use of names and concepts: energy bands, theory of solid bodies; the periods of optimism in the history of physics; the "accidental" change of fields because of Nazism (Hans Bethe). Significant papers on absorption spectra, diamagnetism, phase changes and statistical foundations, published in the 1930s: reasons for selecting these topics; inspirational contributions in the work; responses; associated circumstances and events. Lev Landau and his work also discussed at length.