  The first decade of RHIC running has established the existence of a strongly
coupled Quark Gluon Plasma (sQGP), a new state of nuclear matter with partonic
degrees of freedom. Theory predicts how transitions to this sQGP depend on the
baryon chemical potential, mu_B, and temperature, T. At low mu_B and high T a
cross-over transition occurs. At high mu_B and low T the transition is of first
order. Hence, at intermediate values, a critical point should occur.
Experimentally we can vary these initial conditions by altering the beam
energy. Thus a beam energy scan (BES) will allow us to explore the QCD phase
diagram close to the QGP-hadron gas boundary and locate such key "landmarks" as
the critical point. Establishing the existence of this critical point would be
a seminal step forwards for QCD physics. I discuss below the physics case for a
BES, and explain why RHIC and the STAR experiment are ideally designed for such
a program.
