This book gives a complete exposition of the present status of the theory of the Boltzmann equation and its applications. The Boltzmann equation, an integrodifferential equation established by Boltzmann in 1872 to describe the state of a dilute gas, still forms the basis for the kinetic theory of gases. It has proved fruitful not only for the study of the classical gases Boltzmann had in mind, but also, properly generalized, for electron transport in nuclear reactors, photon transport in superfluids, and radiative transport in planetary and stellar atmospheres. The text presents a unified approach to the problems arising in these different fields, by exploiting similarities whenever they exist and underlining the differences when necessary. But the main exposition is tied to the classical equation established by Boltzmann. Hence the detailed description of applications refers almost exclusively to monatomic neutral gases. Appropiate references are given to papers dealing with similar problems arising in other fields, with particular concern for neutron transport. A unique feature is the detailed consideration of the boundary conditions to be used in connection with the Boltzmann equation. Other topics covered in detail are the derivation of the Boltzmann equation from first principles, the theory of the linearized Boltzmann equation, the use of model equations, and the various regimes of rarefied gas dynamics. In addition to updating the material to 1987, the main improvement over the previous book of the author, "Theory and Application of the Boltzmann equation" is the detailed survey of the use of the techniques of functional analysis in connection with the nonlinear Boltzmann equation, a subject which has greatly progressed in the last ten years.