The basic concepts underlying the response of materials to ion-beam irradiation are outlined. These include the slowing of energetic ions, the creation of defects, sputtering, ion-beam mixing, the acceleration of kinetic processes, and phase transformations. Several examples are cited to illustrate how each of these concepts can be exploited to modify materials in ways not easily achieved, or not even possible, by more conventional processing methods. The chapter attempts to provide a physical understanding of the basic effects of ion-beam irradiation on materials, to enable readers in other areas of research to better understand the more technical chapters that follow, and to develop ideas relevant to their own disciplines. We provide references to more quantitative treatments of the topics covered here.