Relativistic heavy-ion collisions probe the properties of extremely high temperature matter (>10^12 degrees), where there is expected to exist a new phase of matter called the Quark-Gluon Plasma. These experiments indicate the presence of a substance that behaves as a strongly-coupled, almost-perfect fluid. I review some of the key results that indicate this strong collective behavior, and outline the
current progress toward quantifying properties of the system, such as the shear viscosity of the Quark-Gluon Plasma.