Abstract
Superfluidity of a two-dimensional system of excitonic polaritons in an optical microcavity with embedded quantum well is considered. By using the effective low-energy action for thermodynamic phase fluctuations we obtain the expression for the analogue of superfluid density in the system in terms of the "current-current" correlation function. The expression for the "current" operator is also derived. Using the Weakly interacting Bose gas approximation we calculate the "superfluid density" in a polaritonic system and discuss the Kosterlitz-Thouless phase transition in the system.