Abstract Various theoretical arguments have suggested that exotic states of matter may be formed at sufficiently high energy densities ( E > 1–3 GeV/fm 3). Under such extreme conditions, which may have been present very shortly after the Big Bang, quarks and gluons would become deconfined forming a quark-gluon plasma (QGP), which would subsequently transform to hadronic and then to nuclear matter. To explore the creation of such new states of matter in the laboratory, and to study their fundamental properties, the Relativistic Heavy Ion Collider (RHIC) is being constructed at Brookhaven National Laboratory. RHIC will accelerate two counter-rotating beams of heavy ions (e.g., Au) to energies of 100 GeV/u each, which will then intersect producing ultra-high energy collisions possibly leading to effective temperatures of 10 12–10 13 K and densities 5–10 times normal nuclear density. The STAR Collaboration has been formed to design, build, and operate a detector system for RHIC, which will have sufficiently broad capabilities to search for signatures of the QGP and to investigate the behavior of strongly interacting matter at high energy density. Some of the design criteria and the major features of the STAR Detector are described.