In this paper we deal with several characterizations of the Hardy-Sobolev spaces in the unit ball of \mathbb C^n , that is, spaces of holomorphie functions in the ball whose derivatives up to a certain order belong to the classical Hardy spaces. Some of our characterizations are in terms of maximal functions, area functions or Littlewood-Paley functions involving only complex-tangential derivatives. A special case of our results is a characterization of H^p itself involving only complex-tangential derivatives.