  $P$-brane solutions of low-energy string actions have traditionally provided
the first evidence for the existence of string dualities, in which fundamental
and solitonic $p$-branes are identified with perturbative and non-perturbative
BPS states. In this talk we discuss the composite nature of solutions, which
allows for the interpretation of general solutions as bound states or
intersections of maximally supersymmetric fundamental constituents. This
feature lies at the heart of the recent success of string theory in reproducing
the Beckenstein-Hawking black hole entropy formula.
