answer (whatever it is) has been named ‘M theory’. Some people believe that M theory is more fundamental than the five superstring theories in 10d, but I do not share that viewpoint. Rather, for reasons that will be explained in these lectures, I believe that it is on a roughly equal footing with the type IIB superstring theory (or ‘F theory’). Each of these descriptions (extended by various possible compactifications) gives access to different ‘patches’ of the space of quantum vacua. A good analogy, which I heard first from Vafa (at the CERN workshop in June 1996), is that the underlying theory is being defined in much the same way that one defines a manifold. A manifold can be defined by giving a covering by open sets that are diffeomorphic to open sets in $\RR^n$ and by consistently defining transition functions on their overlaps. In the proposed analogy, each of the superstring theories corresponds to one of the open sets, and the dualities that characterize their non-perturbative equivalences correspond to the transition functions. From this viewpoint the various dualities could be viewed as part of the [*definition*]{} of the underlying theory rather than as conjectured theorems that require proof. Of 