S-matrix theory was an ambitious program to constrain the possible scattering amplitudes of Poincare invariant QFTs based on a list of basic tenets expected on physical grounds. The techniques developed in this program have become relevant again because of their adaptation to correlators of CFTs. In this project we have studied some aspects of S-matrix theory that have found recent applications. In this report we review some of technical details of the basic assumptions, singularity structures, high energy behaviour, analyticity in angular momentum etc of scattering amplitudes.