The textbooks "Solid State Theory" give an introduction to the methods, contents and results of modern solid state physics in two volumes. This first volume has the basic courses in theoretical physics as prerequisites, i.e. knowledge of classical mechanics, electrodynamics and, in particular, quantum mechanics and statistical physics is assumed. The formalism of second quantization (occupation number representation), which is needed for the treatment of many-body effects, is introduced and used in the book. The content of the first volume deals with the classical areas of solid state physics (phonons and electrons in the periodic potential, Bloch theorem, Hartree-Fock approximation, density functional theory, electron-phonon interaction). The first volume is already suitable for Bachelor students who want to go beyond the basic courses in theoretical physics and get already familiar with an application area of theoretical physics, e.g. for an elective subject "Theoretical (Solid State) Physics" or as a basis for a Bachelor thesis. Every solid-state physicist working experimentally should also be familiar with the theoretical methods covered in the first volume. The content of the first volume can therefore also be the basis for a module "Solid State Physics" in the Master program in Physics or, together with the content of the 2nd volume, for a module "Theoretical Solid State Physics" or "Advanced Theoretical Physics". The following second volume covers application areas such as superconductivity and magnetism to areas that are current research topics (e.g. quantum Hall effect, high-temperature superconductivity, low-dimensional structures).