Abstract Marco Ademollo, in his contribution to this Volume, has given a review of early work that led to string theory. I will complement this view of that period and its origin. My main purpose is to go further back in time and put the appearance of string theory work in the context of the evolution of the theory of elementary particles. Introduction Just after the end of the second world war, the resounding confirmation of relativistic field theory (more precisely, quantum electrodynamics) took place with Lamb's measurement of the level shifts in hydrogen and Schwinger's calculation of the electron magnetic moment, which differed from the value predicted from Dirac's equation. Renormalization theory with American, European, and Japanese contributions gave remarkable results with accuracies beyond one part in one hundred million. All of these results were obtained using perturbation theory and though the agreement was remarkable, questions of principle on whether the series are convergent and divergences exist, requiring a modification of the theory, loomed in the discussions. Soon after, with the discovery of mesons, and later other baryons, besides the proton and neutron, attention concentrated on the strong interactions. Pauli, Wentzel, Tamm, Dancoff, Dyson and others worked on the problem (for a short review, see Salpeter [Sal08]). The natural thing to do was to write a Lagrangian for mesons and nucleons, and try all possible couplings in perturbation theory. It soon became clear that these calculations did not describe the experimental data.