The desire for crystalline-like electron transport and amorphous (glass)-like heat transport is encapsulated by the "phonon glass electron crystal" (PGEC) concept originally put forward by Slack. Among the most widely-studied PGEC materials are the skutterudites and inorganic clathrates, which are "cage" compounds with large intrinsic cavities that can incorporate loosely-bound guest ions to act as phononscattering centres while electronic transport occurs through the crystalline host framework.