Electron density is one of the fundamental concepts underpinning modern chemistry, and this book presents a conceptual treatment of the subject with equal emphasis on computational and philosophical questions. Beginning with the use of probabilities in statistical physics and the origins of quantum mechanics, the conceptual and statistical framework developed is employed to treat quantum entanglement, Bader's theory of atoms in molecules, open systems, and the theory of insulators. Employing a coherent and logical structure, the book is accessible to students and faculty working in chemistry and computational materials science.