Reactor physics is the study of the neutron and its behavior. Neutrons and protons are the building blocks of the material world. Reactor physics can be used to study how neutrons and gamma rays scatter off of atomic nuclei and how they lose kinetic energy in the process. As an alternative to neutron diffusion theory and neutron transport theory, the behavior of individual neutrons can be described by a statistically based approach called the Monte Carlo method, which was originally developed from the study of game theory. The chapter discusses the types of particles that are important to the behavior of a nuclear power plant as a whole. In addition to Newton's Laws of Motion, the Laws of Electromagnetism, and Einstein's formulation of Special and General Relativity, there are three other conservation laws that all particles in a nuclear reactor must obey: the conservation of mass/energy, momentum, and charge.