This chapter deals with collisions between two particles characterized by an energetic projectile striking a stationary target. Three categories of projectiles of interest in medical physics are considered: light charged particles such as electrons and positrons, heavy charged particles such as protons and α particles, and neutral particles such as neutrons. The targets are either atoms as a whole, atomic nuclei, or atomic orbital electrons. The collisions are classified into three categories: (1) Nuclear reactions, (2) Elastic collisions, and (3) Inelastic collisions.