  The actions, anomalies, and quantization conditions allow the M2-brane and
the M5-brane to support, in a natural way, structures beyond Spin on their
worldvolumes. The main examples are twisted String structures. This also
extends to twisted String^c structures, which we introduce and relate to
twisted String structures. The relation of the C-field to Chern-Simons theory
suggests the use of the String cobordism category to describe the M2-brane.
