Spectroscopy is the chemist’s most powerful tool for observing atomic and molecular structure and behaviour of atoms and molecules. We will use quantum chemistry to describe electronic spectroscopy using simple models such as “electrons in a box”, as well as atomic and molecular orbital theory. The relationship between microscopic interactions and bulk thermodynamic properties will then be examined and the principles of statistical mechanics developed. Various models of bonding are introduced and applied to compounds of the transition metals, providing a basis for understanding the special chemistry of these fascinating elements. Stereochemistry, isomerism, spectroscopy and magnetic properties are studied, and the essential role played by transition metals in biological and catalytic systems is highlighted.