This book provides a balanced and integrated introduction to the quantum world of atoms and molecules. The underlying basis of quantum mechanics is carefully developed, with respect for the historical tradition and from a molecular angle. The fundamental concepts in the theory of atomic and molecular structure are thoroughly discussed, as are the central techniques needed in quantum-chemical applications. Special attention is paid to exposing and clarifying the common ground of Hartree-Fock theory and density-functional theory. Throughout the text, the discussion is pedagogically obliging and aims at simplicity and mathematical clarity, while avoiding the use of advanced mathematics. End-of-chapter problems supplement the main text.