Historically, relativistic string theory was developed as a possible theory of strong interactions before the role was filled by quantum chromo-dynamics. Subsequently, the occurrence of a massless spin-2 particle in the spectrum of states of the string suggested an alternative use for string theory as a possible framework for gravitation that might succeed in overcoming the problems that had been encountered in trying to obtain a renormalizable theory of quantum gravity. An action is required to embody the dynamics of the bosonic string. Because the bosonic string possesses gauge symmetries, namely world sheet reparametrization invariance, the procedure required for path integral quantization of the string is very similar to the Faddeev—Popov method required for the quantization of gauge field theories.