Generalized Action Principle for superstrings and supermembranes

Two approaches concerning the connection of the fermionic kappa -- symmetry with the superstring world -- sheet superdiffeomorphism transformations are discussed. The first approach is based on the twistor -- like formulation of the superstring action and the second one on a reformulation of the superstring and super~-~p~-~brane actions according to the Generalized Action Principle.