As College Tutors we find that a major difficulty in teaching statistical thermodynamics to chemistry students is that (somewhat surprisingly) when they first encounter this subject they often find the underlying physical principles more difficult to grasp than the mathematical formalism. Thus the question 'What is a partition function?' is usually met with blank incomprehension, even though the equation defining this function is well-known and its derivation can be reproduced on request. A number of existing textbooks provide excellent coverage of the more formal aspects of the subject and examine the philosophical foundations on which it rests. In this book, however, one of our principle aims is to make clear to the beginner the physical basis of the relation between the observable behaviour of systems of atoms or molecules and the properties of the individual particles. Some acquaintance with elementary calculus and quantum theory is assumed, as is a familiarity with the basic ideas of classical thermodynamics.