Collisionless plasmas are a common feature of many astrophysical systems (e.g. active galactic nuclei, supernova remnants, clusters of (galaxies) and exhibit unique properties stemming from the loose coupling between particle species and the plasma instabilities that can accelerate some species efficiently and others not at all. Supernova remnant shocks are efficient particle accelerators and may be responsible for the bulk of cosmic rays below 1000 TeV. Galactic and Magellanic Cloud remnants are close enough that the Chandra X-ray observatory has enabled us to spatially resolve theshock heated material as it relaxes downstream. A complete picture of the particle acceleration in collisionless shocks requires the examination of the magnetic field, the thermal and cosmic raypopulations of protons, ion and electrons, and the dynamical effects of magnetic field amplification and cosmic ray losses. In this talk I will focus on the diagnostic capabilities of X-ray spectralobservations of the thermal plasma advecting away from the shock to determine not only the initial electron heating at the shock front butalso to see the effects of the elusive cosmic-ray ions.