In the period between the birth of quantum mechanics and the late 1950s, V.A. Fock wrote papers that are now deemed classics. In his works on theoretical physics, Fock not only skillfully applied advanced analytical and algebraic methods, but also systematically created new mathematical tools when existing approaches proved insufficient. This collection of Fock's papers published in various sources between 1923 and 1959 in Russian, German, French, and English. These papers explore some of the fundamental notions of theoretical quantum physics, such as the Hartree-Fock method, Fock space, the Fock symmetry of the hydrogen atom, and the Fock functional method. They also present Fock's views on the interpretation of quantum mechanics and the fundamental significance of approximate methods in theoretical physics. V.A. Fock was a key contributor to one of the most exciting periods of development in 20th-century physics, and this book conveys the essence of that time. The seminal works presented in this book are a helpful reference for any student or researcher in theoretical and mathematical physics, especially those specializing in quantum mechanics and quantum field theory.