  We have investigated the magnetic properties of non-doped ZnO nanostructures
by using {\it ab initio} total energy calculations. Contrary to many proposals
that ferromagnetism in non-doped semiconductors should be induced by intrinsic
point defects, we show that ferromagnetism in nanostructured materials should
be mediated by extended defects such as surfaces and grain boundaries. This
kind of defects create delocalized, spin polarized states that should be able
to warrant long-range magnetic interactions.
