This original 2018 work, based on the author's many years of teaching at Harvard University, examines mathematical methods of value and importance to advanced undergraduates and graduate students studying quantum mechanics. Topics include the Fourier transform, the spectral theorem for bounded self-joint operators, unbounded operators and semigroups, Weyl's theorem, the Rayleigh-Ritz method, one dimensional quantum mechanics, Ruelle's theorem, scattering theory, and many other subjects. AUTHOR: Shlomo Sternberg received his Ph.D. from Johns Hopkins University in 1957 and taught in the Mathematics Department at Harvard University from 1959 until his retirement as Professor Emeritus of Mathematics in 2017. He is the author of many frequently cited and highly regarded books in mathematics and related fields, including two published by Dover: Dynamical Systems and Curvature in Mathematics and Physics. Among many other honors and awards, Dr. Sternberg was elected to the American Academy of Arts and Sciences in 1984 and to the National Academy of Sciences in 1986.