This book describes, as simply as possible, the mechanisms of scattering (both elastic and inelastic) of electrons with solid targets (electron-atom, electron-plasmon, and electron-phonon interactions). It also presents the main strategies of the Monte Carlo method, as well as numerous comparisons between simulation results and the experimental data available in the literature. Furthermore it provides readers with all the information they need in order to write their own Monte Carlo code and to compare the obtained results with the many numerical and experimental examples presented throughout the book. An extended and updated third edition of a work published in 2014 (first edition) and in 2017 (second edition) on the application of the Monte Carlo method to the transport of fast electrons in solids, this book includes, as novel topics, the theory of polarized electron beams (i.e. density matrix and spin polarization), the study of elastic scattering by molecules, a classical treatment of the Bethe-Bloch stopping power, a simple derivation of the f- and ps-sum rules, the Vicanek and Urbassek formula for the calculation of the backscattering coefficient, the Wolff theory describing the secondary electron spectra, and fundamental aspects of the interactions between electrons beams and solid targets. Further, it describes a completely analytical approach (the so-called multiple reflection method) for calculating the absorbed, backscattered, and transmitted fractions of electrons from unsupported and supported thin films. It also discusses recent applications of the Monte Carlo method.