This chapter discusses general properties of the radiation fields at accelerators. The charged particles directly accelerated, and otherwise manipulated by the electromagnetic fields within the accelerator, are referred to as the primary particles or primary beam. All other particles that are produced from this beam either result from interactions of these primary particles in matter or are due to synchrotron radiation and are referred to as secondary particles or secondary beam. The primary tool for determining the amount of radiation reaching a given location is the stationary form of the Boltzmann equation, or more simply the Boltzmann equation. The Monte Carlo method is the most common approach in radiation physics to solving the Boltzmann equation for realistic geometries that are difficult to characterize using analytic techniques. The method proceeds by constructing a series of trajectories or histories, choosing each step at random from a distribution of applicable processes described as realistically as possible.