The document introduces the Keldysh technique, which is used to calculate non-equilibrium Green's functions. It discusses equilibrium Green's functions, the Keldysh contour used to calculate non-equilibrium Green's functions, and the application of the Keldysh technique. Key aspects covered include Green's functions on the Keldysh contour, the Larkin representation, identities among Green's functions, equations of motion for Green's functions on the Keldysh contour, and Keldysh Green's functions. The goal is to provide an overview of the Keldysh technique.