Collisions of heavy nuclei at very high energies explore the phase transformation from hadronic to partonic degrees of freedom which is predicted to occur at several times normal nuclear density and/or for temperatures in excess of a trillion Kelvin. Such a state, often referred to as a quark-gluon plasma (QGP), is thought to have been the dominant form of matter in the universe in the first few microseconds after the Big Bang. Data from Brookhaven National Laboratory’s Relativistic Heavy Ion Collider (RHIC) and more recently the Large Hadron Collider (LHC) clearly demonstrate that a) these very high temperatures and densities have been achieved, b) the matter does not behave as a quasi-ideal state of free quarks and gluons and instead c) the matter is a dense fluid with very low kinematic viscosity, approaching a conjectured viscosity bound obtained via the gauge/gravity duality. As such, the “strongly-coupled QGP” may be the most perfect fluid ever studied in the laboratory.