The evolution of linear free energy relationships is traced from the Bronsted equation for general acid-base catalysis (1924) to the Hammett rho-sigma equation for the effects of meta or para substituents on the rate or equilibrium constants of side-chain reactions of benzene derivatives (1937). In addition to a natural emphasis on Louis P. Hammett´s own contributions, those of G.N. Burkhardt and his colleagues are also described, along with the extent to which these two authors interacted.