  Some relationships between string theories and eleven-dimensional
supergravity are discussed and reviewed. We see how some relationships can be
derived from others. The cases of N=2 supersymmetry in nine dimensions and N=4
supersymmetry in four dimensions are discussed in some detail. The latter case
leads to consideration of quotients of a K3 surface times a torus and to a
possible peculiar relationship between eleven-dimensional supergravity and the
heterotic strings in ten dimensions. Lecture given at "S-Duality and Mirror
Symmetry", Trieste, June 1995.
