The objective of this investigation is to study the composition, energy spectra and spatial distribution of energetic particles, primarily in interplanetary space. The instrument consists of four identical telescopes inclined at angles of 22.5, 67.5, 112.5, and 157.5 degrees with respect to the spacecraft spin axis. Each telescoope has a geometric factor of about 0.08 cm^2sr and a field-of-view with a full angle of 35 degrees. The multiple sensors and spacecraft spin allow 80% directional coverage for three-dimensional resolution of anisotropies. Energy ranges are 0.5 - 1.5 MeV for protons, 0.1 - 0.5 MeV/nucleon for Z>=1 ions, and 0.4 - 4 MeV/nucleon for heavy (e.g., He, O, S, Fe) ions. Clear compositional separation is achieved for hydrogen, helium and the heavier nuclei up to iron. Lower resolution channels are provided for 0.1 - 0.4 and >0.3 MeV electrons. The "E-dE/dx" technique is used to determine particle composition and energy. Particles must pass through a very thin (five micron epitaxial silicon) front detector before stopping in a thick second detector. High energy penetrating particle events are vetoed by a third, anticoincidence detector. All four telescopes operate in a self-calibrating mode, where particle tracks in pulse height matrices can be used to obtain absolute calibrations. The detector stack is surrounded by a massive platinum shield for exclusion of side-penetrating events, and the front detectors are protected against sunlight by 80 microgram/cm^2 layers of aluminum. Command and data channels are shared with the Interstellar Neutral Gas experiment. The instrument is fully described in Keppler, E., et al., Astron. Astrophys. Suppl. Ser., 92(2), 317-331, Jan. 1992 (NSSDC TRF ID B40458-000A).