We characterize finite index depth 2 inclusions of type II_1 factors in terms
of actions of weak Kac algebras and weak C*-Hopf algebras. If N\subset M
\subset M_1 \subset M_2 \subset ... is the Jones tower constructed from such an
inclusion N\subset M, then B=M^\prime \cap M_2 has a natural structure of a
weak C*-Hopf algebra and there is a minimal action of B on M_1 such that M is
the fixed point subalgebra of M_1, and M_2 is isomorphic to the crossed product
of M_1 and B. This extends the well-known results for irreducible depth 2
inclusions.
