  The progress over the 30 years since the first high-energy heavy-ion
collisions at the BNL-AGS and CERN-SPS has been truly remarkable. Rigorous
experimental and theoretical studies have revealed a new state of the matter in
heavy-ion collisions, the quark-gluon plasma (QGP). Many signatures supporting
the formation of the QGP have been reported. Among them are jet quenching, the
non-viscous flow, direct photons, and Debye screening effects. In this article,
selected signatures of the QGP observed at RHIC and the LHC are reviewed.
