String Theory and M-Theory Superstrings are widely thought to provide the basis for a unified theory of all of physics. Superstrings propagate in ten dimensions and yield a consistent perturbative quantum theory of gravity along with Yang-Mills gauge interactions. At low energy superstrings behave as particles and their low energy effective actions are the ten-dimensional supergravity theories. In the perturbative formulation of string theory, spacetime scattering amplitudes are computed using a conformal quantum field theory (CFT) living on the string world-sheet. The lightest modes of these strings give rise to gravity and gauge forces but strings also include an infinite tower of massive modes. As such the familiar geometrical notion of spacetime is not built into the theory from the start but is derived from its low-energy spectrum. However we now know that the notions of particles and strings should be generalised to include other higher-dimensional extended objects known as branes. Furthermore there is a deeper underlying theory, known as M-theory, whose microscopic dynamics is as yet unknown but which unifies the various string theories and contains M2-branes and M5-branes but no particles or strings.