Over the past few decades, the excitation and ionization of atmospheric gases has become an area of intense research. A large amount of data have been accumulated concerning the various elementary processes which occur when photons, electrons and ions collide with atoms and molecules. This scattered information has now been collected in a handbook for the first time, and the authors give a critical analysis of relevant data. This book is a comprehensive and detailed study of the available information and is distinguished by the following outstanding features: the consideration of a large number of atmospheric constituents, including H^O2, H, N^O2, N, O^O2, O, CO, CO^O2, H^O2O, HCl and some hydrocarbons the maximum number of space particles, including magnetospheric particles, are considered as projectiles: photons, electrons, hydrogen atoms, protons and helium ionsthe energy range under study corresponds to the real spectrum of cosmic fluxes, from threshold values for elementary processes up to several thousand keV the recommended values of cross sections, obtained from analysis of the available experimental data, are given in the handbook and their accuracy is estimated. These features make the handbook particularly valuable to specialists in the aeronomy of planets, comets and active perturbations, as well as to experimentalists and theoreticians working in the fields of plasma physics, atomic and molecular physics, physics of the upper atmosphere, chemical physics, optics and spectroscopy.