This work represents a summation of our current state of understanding of trapped particle environment. It focuses on the region of the magnetosphere where charged particles are trapped in closed orbits by the Earth's magnetic field. Topics include characteristics of the trapped particle population, dynamics, energization processes, source and loss mechanisms, the effects of trapped particles on satellite systems, and current efforts at modeling and predicting the trapped particle environment. It is intended for researchers in space plasma physics and particle astrophysics, engineers and satellite operators.