The Joint European X-ray Telescope, JET-X, is one of the core instruments in the scientific payload of the USSR''s Spectrum Roentgen-Gamma (RG) high energy astrophysics mission. JET-X consists of two identical co-aligned X-ray imaging telescopes, each with a spatial resolution of 20 arc second. Focal plane imaging is achieved with cooled X-ray sensitive CCD detectors, which provide high spectral resolution and good background rejection efficiency, in addition to the necessary imaging capability. An optical monitor telescope, also co-aligned with the two X-ray telescopes, permits simultaneous observation and identification of optical counterparts of X-ray target sources. The system design of JET-X is reviewed, and performance data obtained from measurements on the instrument prototype are presented.