  Most of our knowledge of the physical processes in distant plasmas is
obtained through measurement of the radiation they produce. Here we provide an
overview of the main collisional and radiative processes and examples of
diagnostics relevant to the microphysical processes in the plasma. Many
analyses assume a time-steady plasma with ion populations in equilibrium with
the local temperature and Maxwellian distributions of particle velocities, but
these assumptions are easily violated in many cases. We consider these
departures from equilibrium and possible diagnostics in detail.
