This book focuses on the basic concepts in molecular reaction dynamics, which is the microscopic atomic-level description of chemical reactions, in contrast to the macroscopic phenomenological description known from chemical kinetics. It is a very extensive field and we have obviously not been able, or even tried, to make a comprehensive treatment of all contributions to this field. Instead, we limited ourselves to give a reasonable coherent and systematic presentation of what we find to be central and important theoretical concepts and developments, which should be useful for students at the graduate or senior undergraduate level and for researchers who want to enter the field.