Energetic particles in solids lose their energy partly to atoms and ions, by elastic collisions, and partly to electronic excitations. Most of the present chapter describes the way in which energetic particle beams generate electronic excitation. We shall begin by considering the interaction of charged particles with solids. We include the effects of X-ray, γ-ray and neutron irradiation, since these lead to secondary electrons or ions. In discussing the basics of particle beam interactions with solids, our emphasis is on identifying situations where there is reason to believe that electronic excitation is an essential feature. We shall say relatively little about those situations which are described by elastic collisions, and for which the electrons merely provide a potential to define the interactions between atoms. Thus we shall not discuss certain areas of major industrial significance, such as the doping of semiconductors by ion implantation, for which electronic excitation is a secondary feature. However, we should be aware of the energies and doses which are used in these applications, since they will influence the extent to which excitation is an issue. In ion implantation, the critical parameters are chosen to produce a specific concentration profile for a specific need. For example, doping polysilicon uses high doses (typically 1016 ions/cm2) whereas the adjustment of threshold voltages uses low doses (perhaps 1012 ions/cm2), even though both use similar energies, perhaps 50 keV.