This book deals with a central topic at the interface of chemistry and physics — the understanding of how the transformation of matter takes place at the atomic level. Building on the laws of physics, the book focuses on the theoretical framework for predicting the outcome of chemical reactions. The style is highly systematic with attention to basic concepts and clarity of presentation. The emphasis is on concepts and insights obtained via analytical theories rather than computational and numerical aspects. Molecular reaction dynamics is about the detailed atomic-level description of chemical reactions. Based on quantum mechanics and statistical mechanics, the dynamics of uni- and bi-molecular elementary reactions are described. The book features a comprehensive presentation of transition-state theory which plays an important role in practice, and a detailed discussion of basic theories of reaction dynamics in condensed phases. Examples and end-of-chapter problems are included in order to illustrate the theory and its connection to chemical problems. The second edition includes updated descriptions of adiabatic and non-adiabatic electron-nuclear dynamics, an expanded discussion of classical two-body models of chemical reactions, including the Langevin model, additional material on quantum tunnelling and its implementation in Transition-State Theory, and a more thorough description of the Born and Onsager models for solvation.