Understanding the nature of radiation damage in materials is of paramount importance for controlling the safety of nuclear reactors, semiconductor technology, and designing reliable devices in space. For more than 60 years, the standard approach to estimating the radiation damage in materials analytically has been a simple equation known as Kinchin-Pease. However, the displacements-per-atom (DPA) number obtained from this equation does not usually correspond to any physically measurable quantity in common metals. This was established experimentally about 40 years ago, and computer simulations carried out during the last 25 years have firmly established the physical reason for this.