 String theory has undergone several mini-revolutions over the last several years, one of which involved the discovery of various duality relations, mathematical transformations connecting, in this case, what appear to be physically distinct string theories — type I, type IIA, type IIB, (heterotic) SO(32) and (heterotic) E8×E8 — to one another and to eleven-dimensional supergravity (a particle theory). The discovery of these connections led to the conjecture that all of the string theories are really aspects of a single underlying theory, which was given the name ‘M-theory’ (though M-theory is also used more specifically to describe the unknown theory of which eleven-dimensional supergravity is the low energy limit). The rationale, according to one kind of duality (S-duality), is that one theory at strong coupling (high energy description) is physically equivalent (in terms of physical symmetries, correlation functions and all observable content) to another theory at weak coupling (where a lower energy means a more tractable description), and that if all the theories are related to one another by dualities such as this, then they must all be aspects of some more fundamental theory. Though attempts have been made, there has been no successful formulation of this theory: its very existence, much less its nature, is still largely a matter of conjecture.