Ion irradiations have been critical to the development of both our fundamental and applied understanding of radiation effects. As discussed in Sections 1.07.2 and 1.07.3, it is the flexibility of such irradiations and our firm understanding of atomic collisions in solids that afford them their utility. Principally, ion irradia­tions have enabled focused studies on the isolated effects of primary recoil spectrum, defect displace­ment rate, and temperature. In addition, they have provided access to the fundamental properties of point defects, defect creation, and defect reactions. In this section, we highlight a few key experiments that illustrate the broad range of problems that can be addressed using ion irradiations. We concentrate our discussion on past ion irradiations studies that have provided key information required by modelers in their attempts to predict materials behavior in existing and future nuclear reactor environments, and particularly information that is not readily available from neutron irradiations. In addition, we include a few comparative studies between ion and neutron irradiations to illustrate, on one hand,
the good agreement that is possible, while on the other, the extreme caution that is necessary in extrapolating results of ion irradiations to long-term predictions of materials evolution in a nuclear environment.