  We review the evidence for the various dualities amongst the five D=10
superstring theories and for the existence of M-theory using the associated
effective supergravity theories. We also summarise the combinatorial technics
developed for constructing BPS solutions in D=11 supergravity theory and
conjecture that all the BPS solutions of D<11 supergravity theories can be
derived from the BPS solutions of D=11 supergravity that preserve 1/2 the
supersymmetry. To demonstrate this, we derive the dyonic p-brane solutions from
eleven dimensions.
