This book explores the way in which quantum theory has become central to our understanding of the behaviour of atoms and molecules. It looks at the way in which this underlies so many of the experimental measurements we make, how we interpret those experiments and the language which we use to describe our results. It attempts to provide an account of the quantum theory and some of its applications to chemistry. This book is for researchers working on experimental aspects of chemistry and the allied sciences at all levels, from advanced undergraduates to experienced research project leaders, wishing to improve, by self-study or in small research-orientated groups, their understanding of the ways in which quantum mechanics can be applied to their problems. The book also aims to provide useful background material for teachers of quantum mechanics courses and their students.