  We show that Dirichlet-branes, extended objects defined by mixed
Dirichlet-Neumann boundary conditions in string theory, break half of the
supersymmetries of the type~II superstring and carry a complete set of electric
and magnetic Ramond-Ramond charges. We also find that the product of the
electric and magnetic charges is a single Dirac unit, and that the quantum of
charge takes the value required by string duality. This is strong evidence that
the Dirchlet-branes are intrinsic to type II string theory and are the
Ramond-Ramond sources required by string duality. We also note the existence of
a previously overlooked 9-form potential in the IIa string, which gives rise to
an effective cosmological constant of undetermined magnitude.
