Most scientific studies of imperfect solids concentrate on the properties of individual isolated defects. Such properties include their electronic structure, as shown by optical and spin‐resonance data, formation energies, and diffusion parameters. Yet these properties alone do not always determine those practical applications of solid‐state physics that exploit the behavior of defects. The principal extra ingredients can often be described as defect processes—how defects interact with each other and how the imperfect lattice evolves. That is, the time‐dependent behavior of the imperfect solid, not just its static properties, affects how it can be used.