The goal of heavy ion collisions at high energies is to create a deconfined state of strongly interacting matter, the Quark-Gluon Plasma (QGP), in the laboratory and to study its properties. One approach to study the matter in heavy ion collisions is to the use high-energy partons that are produced in the early stages of the collision, which interact with the Quark-Gluon Plasma as they leave the interaction area. I will present the main of the experimental observations of such 'hard probes' from heavy ion collisions at the LHC and the Relativistic Heavy Ion Collider (RHIC) at lower energies. The emphasis of the presentation will be on explaining how the results inform us about the interactions between high-energy partons and the QGP and the density of produced matter, using both illustrative calculations and comparisons to complete perturbative QCD based calculations of the interactions between high-energy partons and the Quark-Gluon Plasma.