Electron emission from surfaces is of crucial importance in determining the properties of electrical discharges, where it is caused by the impact of ions, atoms, electrons, and photons. At the microscopic level, the physics of electron emission from solid targets is usually described by a three-step model: electron excitation, electron transport to the surface, and its escape through the surface. This division serves a heuristic purpose and it must be borne in mind that, in purely surface events, actually only one step is involved. What distinguishes electron emission by different incoming particles is the excitation step [1], which depends strongly on the momentum of the particle. This is because there is a minimum energy needed to extract an electron from the solid (work function for metals, band gap plus electron affinity for nonmetals) and this energy transfer implies a momentum transfer from the projectile.This chapter is concerned with heavy particle collisions (ions and atoms) at low energies (below a few keV) and will discuss first the physical mechanisms and then applications to electrical discharges in gases. The overall picture of electron emission induced by heavy particles is the following. Electrons are excited from the target or the projectile as a result of Coulomb interactions involving the nuclei and electrons through mechanisms that are grouped in two categories, potential and kinetic, depending on the source of the excitation energy. Such excitations occur mostly in binary collisions at the surface or very shallow depths, since the penetration depth of low-energy atomic projectiles is usually very shallow, tens of nm or less. The excited electrons can be ejected directly into vacuum or undergo a series of collisions in the target solid (electron transport) on their way to the surface. The collisions are either elastic scattering with atomic cores, which cause large deflections in trajectories, or energy loss collisions by scattering with other electrons, contingent on the availability of electronic states. As a result of such inelastic scattering, the electrons which succeed in escaping the solid come from a shallow depth, of the order of 2 nm for metals and semiconductors and up to a few tens of