Certain interactions, such as nuclear forces and the forces of 'high-energy' physics, which arise in the theory of elementary particles, cannot be described successfully by quantum field theory. Considerable interest has therefore centred on attempts to formulate interactions between elementary particles in terms of the S-Matrix, an operator introduced by Heisenberg which connects the input and output of a scattering experiment without seeking to give a localized description of the intervening events. In this book four authors, who are together responsible for many of these developments, set...