  At the interface of two two-dimensional quantum systems, there may exist
interface currents similar to edge currents in quantum Hall systems. It is
proved that these interface currents are macroscopically quantized by an
integer that is given by the difference of the Chern numbers of the two
systems. It is also argued that at the interface between two time-reversal
invariant systems with half-integer spin, one of which is trivial and the other
non-trivial, there are dissipationless spin-polarized interface currents.
