  Solid state quantum condensates can differ from other condensates, such as
Helium, ultracold atomic gases, and superconductors, in that the condensing
quasiparticles have relatively short lifetimes, and so, as for lasers, external
pumping is required to maintain a steady state. In this chapter we present a
non-equilibrium path integral approach to condensation in a dissipative
environment and apply it to microcavity polaritons, driven out of equilibrium
by coupling to multiple baths, describing pumping and decay. Using this, we
discuss the relation between non-equilibrium polariton condensation, lasing,
and equilibrium condensation.
