Linear operators in Hilbert space play a fundamental role in the formulation of quantum theory. This book offers a self-contained presentation of the most important tools and methods from Hilbert space theory, with particular focus on the spectral theory of self-adjoint operators. But it also goes further by describing some applications in quantum mechanics, in particular the analysis of Schrödinger operators and quantum scattering theory. The final two chapters are devoted to Mourre’s conjugate operator method and some of its consequences for scattering theory. The text gives complete proofs and includes numerous exercises.