M-theory is an 11-dimensional quantum theory of gravity which, in addition to gravitons and other particle-like excitations, includes extended objects known as membranes and five- branes. Though a complete definition of M-theory is not yet known, it is proposed as a nonperturbative formulation of superstring theory and as such is a compelling candidate for a unified theory of the fundamental particles and forces in Nature. Much has been learned about M-theory through its symmetries and its relation to supergravity and string theory and this has in turn led to important results in superstring theory and quantum gauge theory.