  For a non-superconducting system, the electronic tunneling current through an
insulating barrier is calculated, including interaction effects. The exact
Hamiltonian of the full system is projected onto the subspaces of the "left"
and "right" leads. In the weak tunneling limit the well-known tunneling
Hamiltonian is recovered, along with an additional term. This additional term
originates from the projection of the electron-electron interaction onto each
subsystem and corresponds to correlated tunneling. It is shown that the
tunneling current is determined by---in addition to the single-particle density
of states---the spin-spin and density-density susceptibilities. The signatures
of which have recently been observed in several experiments.
