The need for a compact insulation system for high-voltage equipment has led to the development of a variety of insulating materials, which are combined to form practical insulation systems: solid, composite solid-liquid, and solid-gas insulation. Solid and composite materials are characterized by the fact that they are non-self-restoring; see Chapter 13. The breakdown processes in solid and composite materials may be classified in two broad categories: direct breakdown as measured in new materials and breakdown by incipient fault, typical in practical insulation systems.