  Working in the Wannier representation, we derive an expression for the
orbital magnetization of a periodic insulator. The magnetization is shown to be
comprised of two contributions, an obvious one associated with the internal
circulation of bulk-like Wannier functions in the interior, and an unexpected
one arising from net currents carried by Wannier functions near the surface.
Each contribution can be expressed as a bulk property in terms of Bloch
functions in a gauge-invariant way. Our expression is verified by comparing
numerical tight-binding calculations for finite and periodic samples.
