  The Trojan-Horse method is an indirect approach to determine the energy
dependence of S factors of astrophysically relevant two-body reactions. This is
accomplished by studying closely related three-body reactions under quasi-free
scattering conditions. The basic theory of the Trojan-Horse method is developed
starting from a post-form distorted wave Born approximation of the T-matrix
element. In the surface approximation the cross section of the three-body
reaction can be related to the S-matrix elements of the two-body reaction. The
essential feature of the Trojan-Horse method is the effective suppression of
the Coulomb barrier at low energies for the astrophysical reaction leading to
finite cross sections at the threshold of the two-body reaction. In a modified
plane wave approximation the relation between the two-body and three-body cross
sections becomes very transparent. Applications of the Trojan Horse Method are
discussed. It is of special interest that electron screening corrections are
negligible due to the high projectile energy.
