  Lectures notes for 2004 Les Houches Summer School on "Nanoscopic Quantum
Transport". These lectures contain an introduction to Keldysh formalism for
interacting bosonic and fermionic systems, presented in the functional integral
framework. Covered topics include: kinetic theory, relation to classical
techniques (such as Martin--Siggia--Rose and Fokker--Planck), non--linear sigma
model for disordered fermions, etc.
