  After reviewing some basic concepts of the theory of strongly interacting
matter above nuclear energy density and reviewing some salient results of the
experimental program at the Relativistic Heavy Ion Collider (RHIC), these
lectures explain why the quark-gluon plasma observed in the RHIC experiments
has been called a ``perfect liquid.'' They then give an introduction to some
recent ideas concerning the possible origin of the nearly inviscid nature of
the quark-gluon plasma and discuss the connection between low viscosity and
strong parton energy loss of hot QCD matter.
