  Strange particles have been a very important observable in the search for a
deconfined state of strongly interacting matter, the quark-gluon plasma (QGP),
which is expected to be formed in ultra-relativistic heavy ion collisions. We
review the main experimental observations made at the Super Proton Synchrotron
(SPS) at CERN, Geneva, and at the Relativistic Heavy Ion Collider (RHIC) at
Brookhaven National Laboratory (BNL). The large amount of recently collected
data allows for a comprehensive study of strangeness production as a function
of energy and system size. We review results on yields, transverse mass and
rapidity spectra, as well as elliptic flow. The measurements are interpreted in
the context of various theoretical concepts and their implications are
discussed. Of particular interest is the question whether strange particles are
in any way sensitive to a partonic phase. Finally, a compilation of
experimental data is provided.
