Molecular electronic structure calculations have assumed increasing importance in a wide range of research fields. Applications abound in fields as diverse as molecular electronics and pharmacology, radioastronomy and organic synthesis. The vast majority of contemporary molecular electronic structure studies are performed within the framework of non-relativistic quantum mechanics. However, recent years have seen a growing recognition of the importance of relativistic effects in molecules, especially in systems containing heavy atoms. This has fuelled the development of methods for relativistic molecular electronic structure studies. This book describes the essential theoretical and computational apparatus for a relativistic quantum chemistry.