  Lifting supersymmetric quantum mechanics to loop space yields the
superstring. A particle charged under a fiber bundle thereby turns into a
string charged under a 2-bundle, or gerbe. This stringification is nothing but
categorification. We look at supersymmetric quantum mechanics on loop space and
demonstrate how deformations here give rise to superstring background fields
and boundary states, and, when generalized, to local nonabelian connections on
loop space. In order to get a global description of these connections we
introduce and study categorified global holonomy in the form of 2-bundles with
2-holonomy. We show how these relate to nonabelian gerbes and go beyond by
obtaining global nonabelian surface holonomy, thus providing a class of action
functionals for nonabelian strings. The examination of the differential
formulation, which is adapted to the study of nonabelian p-form gauge theories,
gives rise to generalized nonabelian Deligne hypercohomology. The (possible)
relation of this to strings in Kalb-Ramond backgrounds, to M2/M5-brane systems,
to spinning strings and to the derived category description of D-branes is
discussed. In particular, there is a 2-group related to the String-group which
should be the right structure 2-group for the global description of spinning
strings.
