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This paper is a brief review of low energy soft hadronic physics, startingfrom the invention of the low energy effective range theory in the late 40'sdue to Bethe and Schwinger for nucleon-nucleon scattering, and itsgeneralization to the static Chew-Low model for pion nucleon scattering, to thepresent development of the Lehmann and Weinberg Chiral Perturbation Theories.It is pointed out that a consistent low energy calculation can be achieved withthe incorporation of the unitarity relation in the Chiral Perturbation Theory.

Bethe-Schwinger Effective Range Theory and Lehmann and Weinberg Chiral Perturbation Theories