  Lecture notes presented at Windsor NATO school on "Field Theory of Strongly
Correlated Fermions and Bosons in Low-Dimensional Disordered Systems" (August
2001). The purpose of these lectures is to give a brief modern introduction to
Keldysh non-equilibrium field theory and its classical analog - Doi-Peliti
technique. The special emphasis is put on stressing the analogy between the two
approaches.
