"Theoretical Solid State Physics" provides an introduction to the methods, contents, and results of modern solid state physics in two volumes. The first volume builds on the foundational course in theoretical physics, meaning that knowledge of classical mechanics, electrodynamics, and particularly quantum mechanics and statistical physics is assumed. The essential formalism of second quantization (occupation number representation) for dealing with many-body effects is introduced and utilized in the book. The content of the first volume covers the classical areas of solid state physics (phonons and electrons in periodic potentials, Bloch theorem, Hartree-Fock approximation, density functional theory, electron-phonon interaction). The first volume is already suitable for bachelor's students who wish to delve deeper into an application area beyond the foundational course in theoretical physics.