Cermet fuel elements, when integrated in a cylindrical core along with reflectors and safety and control components, constitute a very rugged reactor assembly capable of delivering hundreds of megawatts of power at power densities of several gigawatts per cubic metre (several megawatts per litre). The cermet fuel is a ceramic uranium oxide or uranium nitride fuel in a refractory metal matrix fuel element. The fuel element is hexagonal with a flat-to-flat dimension of 20 to 30 mm. Coolant channels of ∼l-mm diam are bored along the hexagonal fuel element. A typical cylindrical active core would have a volume of ∼6 × 10−2 m3 (420 mm in height and diameter) with the core, reflectors, control and safety elements, core support, vessel, and reentry shield cone under 2000 kg. Depending on the particular choice of materials and desired performance characteristics, this reactor can operate at an exit temperature of up to 2700 K. The broad applications of this reactor type include steady-state space platform or lunar base power sources, burst power sources—hundreds of megawatts(thermal) power on demand within 100 s for periods of minutes, and other applications. This reactor type offers easy operational control and meets all the safety requirements for launch and reentry. Land-based development and testing can be performed easily because this fuel type has a high fission product and fissile material retention capability. This reactor concept has been developed considerably in the past few decades. The physics design of the cermet fuel reactor is performed utilizing modern computers and computer codes. This design also incorporates developments in metallurgy and fuel performance, as well as new requirements in safety and performance that have been realized since the late 1960s when this concept was first pursued. The basic fuel element is upgraded and integrated in the overall core and reactor subsystem. Two specific applications are illustrated in detail, and expansion to other applications is outlined.