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I review the S-Matrix approach of a quantum field theory and apply it to strong interaction. General properties like analyticity and unitary lead to constraints on amplitudes. Dispersion relations, for instance, allow to compute an analytical function anywhere in the complex plane from only its singularities (poles and branching cuts). By an analytical continuation of the partial wave expansion in the complex angular momentum plane, I show how poles (Regge poles) provide the main contribution to scattering amplitudes. The general formalism is then applied for strong interaction. I present a (almost) comprehensive study of two-to-two reactions (with pion, kaon or photon beam and nucleon target) in a unified approach. The observables include total and differential cross sections, beam and target asymmetries and spin density matrix elements.