  We present a theory for Coulomb drag between two mesoscopic systems which
expresses the drag in terms of scattering matrices and wave functions. The
formalism can be applied to both ballistic and disordered systems and the
consequences can be studied either by numerical simulations or analytic means
such as perturbation theory or random matrix theory. The physics of Coulomb
drag in the mesoscopic regime is very different from Coulomb drag between
extended electron systems. In the mesoscopic regime we in general find
fluctuations of the drag comparable to the mean value. Examples are the
vanishing average drag for chaotic 2D-systems and the dominating fluctuations
of drag between quasi-ballistic wires with almost ideal transmission.
