Written to be the definitive text on the rotational spectroscopy of diatomic molecules, this book develops the theory behind the energy levels of diatomic molecules and then summarises the many experimental methods used to study their spectra in the gaseous state. After a general introduction, the methods used to separate nuclear and electronic motions are described. Brown and Carrington then show how the fundamental Dirac and Breit equations may be developed to provide comprehensive descriptions of the kinetic and potential energy terms which govern the behaviour of the electrons. One chapter is devoted solely to angular momentum theory and another describes the development of the so-called effective Hamiltonians used to analyse and understand the experimental spectra of diatomic molecules. The remainder of the book concentrates on experimental methods. This book will be of interest to graduate students and researchers interested in the rotational spectroscopy of diatomic molecules.