Festkorperprobleme X: Advances in Solid State Physics is a compilation of papers and lectures on semiconductor physics, low temperature physics, thermodynamics, and metal physics of the German Physical Society, Freudenstadt, on April 6-11, 1970. This volume is a collection of 13 papers in English and German on the abovementioned subjects. The book describes some characteristics of the different families of narrow bandgap semiconductors; the result arising from the interaction between free carriers and acoustic waves in solids; and the advances made in the field of modulation spectroscopy. The text further discusses the relations between the state of the photoemitted electrons and the absorption process in the solid. In Chapter 8, applications to various problems in semiconductor physics are dealt with. The Empirical Pseudopotential Method and the theory of phonon dispersion curves from a pseudopotential point of view are also considered. Further examined is the Ginzburg-Landau theory of superconductivity in relation to the probability distribution of the electric field strength of laser light that has a form completely analogous to that of the pair wave function of the theory. The implications of the thermodynamics of point defects in imperfect crystals and the association of foreign ions and vacancies due to their Coulomb interaction, resulting in complexes, are investigated. This book is of interest to electrical engineers, research engineers, professors, and students in theoretical or experimental physics.