For as long as we have been measuring shock waves in interstellar space, we have, whenever possible, leveraged the velocity distribution function to assist in our understanding of the collisionless plasma dynamics. The first measurements of the distribution function hinted at complex dynamics with reflected particles, and now, as our measurement capabilities have further improved we have developed new techniques for extracting information from phase space. Still, open questions remain on the details of the kinetic processes that occur through a shock and dissipate the large bulk kinetic energy of these supersonic flows into accelerating particles and heating the plasma.

This scene setting talk will provide a historical overview of phase space analysis of collisionless shocks, focusing on advancements in simulation and theory. I will highlight efforts to develop phase space diagnostics and focus in particular on recent work utilizing the field-particle correlation technique. By leveraging phase space, different populations of particles can be analyzed simultaneously, giving new perspective on the dynamics and energetics of reflected particles and the subsequent instabilities these particles can generate. This talk will thus argue that with computing power ever increasing and kinetic simulations becoming more sophisticated, phase space stands as an exciting frontier in which we may finally answer long-standing questions on the dynamics and energetics of collisionless shocks.