Nonequilibrium field theory from the 2PI effective action
  Nonperturbative approximation schemes are inevitable even in weakly coupled theories if the nonequilibrium behavior of quantum fields is investigated. The two-particle irreducible (2PI) effective action formalism provides an efficient framework for obtaining resummation schemes both in and out of equilibrium. We briefly review the these techniques and discuss recent findings for nonequilibrium field theories. 
Szabolcs Borsanyi