The Trojan Horse method (THM) is a powerful indirect technique that provides information to determine astrophysical factors for binary rearrangement processes x + A → b + B at astrophysically relevant energies by measuring the cross section for the Trojan Horse (TH) reaction a + A → y + b + B in the quasifree kinematics. The theory of the THM both for direct and resonant binary subreactions is presented based on the few-body approach and Green's function formalism. The off-energy-shell effects and initial and final state interactions have been taken into account. To treat the resonance processes we developed the half-off-energy-shell R matrix approach. Examples are presented to demonstrate how THM works.