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'.