  In this note we follow the historical development of the ideas that led to
the formulation of String Theory. We start from the inspired guess of Veneziano
and its extension to the scattering of $N$ scalar particles, then we describe
how the study of its factorization properties allowed to identify the physical
spectrum making the string worldsheet manifest and finally we discuss how the
critical values of the intercept of the Regge trajectory and of the critical
dimension were fixed to 1 and 26.
