This chapter describes the production of induced radioactivity at accelerators. It deals with a review of the basic principles of the production of radioactivity. The chapter also deals with a discussion of the activation of accelerator components. Induced radioactivity can be produced at nearly all accelerators that produce particles and ions above all but the most minimal energies. Protons having energies above about 10 MeV will produce radioactivity upon interacting with matter. In some special cases, radioactivity can be produced at much lower energies due to exothermic nuclear reactions that either produce radionuclides directly or emit neutrons capable of inducing radioactivity through their secondary interactions. In some configurations it is worthwhile despite the additional costs to minimize the amount of sodium in the concrete ingredients in order to reduce exposures to individuals conducting maintenance on the accelerator.