Unimolecular reactions are in principle the simplest chemical reactions, because they only involve one molecule. The basic mechanism, in which the competition between the chemical reaction step and a collisional deactivation leads to a pressure-dependent coefficient, has been understood for a long time. However, this is a rapidly developing field, and many new and important discoveries have been made in the past decade. This First Part, part of Two CCK Volumes dealing with Unimolecular Reactions, deals with the Reaction Step. The first chapter is an introduction to the whole project, aiming to cover the material necessary to understand the content of the detailed chapters, as well as the history of the development of the area. Chapter 2 is a review of the modern view of the statistical theories, as embodied in the various forms of RRKM theory. Chapter 3 deals with the fully quantum mechanical view of reactive states as resonances. It presents considerable advances in the field made during the last decade. It treats both the statistical as well as the fully quantum mechanical view.