In theoretical physics, the LSZ (reduction) formalism is a method to calculate the S-matrix (the scattering amplitudes) from the time ordered correlation functions of a quantum field theory. It is named for the three german physicists Harry Lehmann , Kurt Symanzik and Wolfhart Zimmermann .

This formalism does not cover bound states or massless particles. However, a generalization of LSZ can cover bound states, provided we deal with composite fields which are often nonlocal.

Also, the LSZ formalism cannot handle topological solitons at all.