The NERVA Experimental Engine (XE) will be ground tested in a test stand at the Nuclear Rocket Development Station, Jackass Flats, Nevada. The nuclear reactor and its associated controls and instrumentation comprise the XE Nuclear Subsystem, a major engine component. This article describes the controls and instrumentation portion of the subsystem and delineates its functions. Principal controllers for power and temperature were developed from earlier models used in the testing of KIWI reactors by Los Alamos Scientific Laboratory. Controllers for automatic startup and override were later developments for use with NERVA Control. Other portions of the subsystem include nuclear instrumentation, a safety system, and disgnostic instrumentation.