 Known variously as string theory, superstring theory, and M-theory, this program (qua theory of quantum gravity) has its roots, indirectly, in the observation, dating back to at least the 1930s, that classical general relativity looks in many ways like the theory of a massless ‘spin-two’ field propagating on the flat Minkowski spacetime of special relativity. [See Cappelli et al. (eds.) 2012, and Gasperini and Maharana (eds.) 2008, for collections of essays covering the early history of string theory; Rickles 2014 offers a conceptually-oriented history of the earlier days of string theory; Rovelli 2001b (Other Internet Resources section below) and 2006 offer a capsule history, and Greene 2000 provides a popular account.] This observation led to early attempts to formulate a quantum theory of gravity by “quantizing” this spin-two theory. However, it turned out that the theory is not perturbatively renormalizable, meaning that there are ineliminable infinities. Attempts to modify the classical theory to eliminate this problem led to a different problem, non-unitarity, and so this general approach was moribund until the mid-1970s, when it was discovered that a theory of one-dimensional “strings” developed around 1970 to account for the strong interaction, actually provided a framework for a unified theory which included gravity, because one of the modes of oscillation of the string corresponded to a massless spin-two particle (the ‘graviton’).