I will describe the particle-based approach to kinetic simulation. A system of N particles can be classified using the number of particles that determine the motion of a given test particle. For plasmas, the radius of interest is the Debye length. When many particles reside in a Debye sphere, the particle in cell approach is most suitable. When few particles are present in a Debye sphere, the molecular dynamics approach is to be preferred. An intermediate approach that combines virtues of both approaches is the particle-particle, particle-mesh (PPPM) method. After briefly describing each approach, I will provide one example, the simulation of structure formation in the universe.