  Effective world-brane actions for solitons of ten-dimensional type IIA and
IIB superstring theory are derived using the formulation of solitons as
Dirichlet branes. The one-brane actions are used to recover predictions of
SL(2,Z) strong-weak coupling duality. The two-brane action, which contains a
hidden eleventh target space coordinate, is shown to be the eleven-dimensional
supermembrane action. It can be thought of as the membrane action of
`M-theory'.
