The High Energy X-Ray Timing Experiment (HEXTE) is one of three x-ray instrument components on NASA's X-Ray Timing Explorer (XTE) mission to be launched in 1994 for the detailed, 2-200 keV study of bright x-ray pulsars and other x-ray sources through observations of their temporal variability. Through extensive, dedicated, pointed observations of these sources, XTE will directly address issues pertaining to the nature of compact matter (white dwarfs to massive black holes), the evolution of systems containing these objects, and the conditions of astrophysical plasmas in their vicinity under extreme conditions of gravity, magnetic fields, and temperatures. The HEXTE is composed of two clusters, each containing four NaI/CsI phoswich scintillation counters with a total net collecting area of 800 cm2 per cluster. The detector system is designed to achieve state-of-the-art scintillator energy resolution (AEA 5. 0.1 @ 100 keV) and high spectral sensitivity (3 σ detection of 1 milliCrab active galaxy in one energy resolution element - Δ E = 20 keV - at 100 keV in 105s). This high performance is achieved through the large area and controlling systematic uncertainties to be 0.001 of instrument background, by utilization of aperture modulation rapid with respect to instrument background variations, automatic gain control, and a small field of view (1° FWHM). Instrument control and data formatting is accomplished by an onboard microcomputer system, which can be configured to maximize the scientific return for each individual observation within the constraint of 5000 bits/s of allocated telemetry.