Particle codes are a powerful tool for the numerical simulation of nonlinear plasma behavior. In these codes, one follows the motion of a large number of electrons and ions in their self-consistent (plus externally imposed) electric and magnetic fields. The fields are solved for on a spatial grid chosen to resolve the collective behavior of the plasma (i.e., the plasma waves). The interpolation between the grid and the particle positions corresponds physically to a multipole expansion of finite-size charges about their nearest grid point location. Results from particle codes agree with numerical solutions of the Vlasov equation. In laser fusion applications, particle codes are used to study the absorption of laser light.