After a brief introduction to the physics of high-energy nuclear collisions, we will present recent experimental results that are closely connected to the properties of the matter produced in Au+Au collisions at RHIC. We will focus on collective observables such as transverse radial flow and elliptic flow. With experimental observations, especially with the multi-strange hadron results, we will demonstrate that collectivity has already been developed prior to the hadronic stage in heavy ion collisions at RHIC. The energy dependence of collective observables will be discussed. Finally, we will develop a plan for future measurements that are needed for characterization of the partonic Equation of State in high-energy nuclear collisions.