This book deals with the classical kinetic theory of gases. Its aim is to present the basic principles of this theory within an elementary framework and from a more rigorous approach based on the Boltzmann equation. The subjects are presented in a self-contained manner such that the readers can understand and learn some methods used in the kinetic theory of gases in order to investigate the Boltzmann equation. This book can be useful as a textbook for students and researchers who are interested in the foundations of the Boltzmann equation and in the classical methods used in the kinetic theory for the determination of the transport coefficients of the gases.