  We show that MHV diagrams are the Feynman diagrams of certain twistor actions
for gauge theories in an axial gauge. The gauge symmetry of the twistor action
is larger than that on space-time and this allows us to fix a gauge that makes
the MHV formalism manifest but which is inaccessible from space-time. The
framework is extended to describe matter fields: as an illustration we
explicitly construct twistor actions for an adjoint scalar with arbitrary
polynomial potential and a fermion in the fundamental representation and show
how this leads to additional towers of MHV vertices in the MHV diagram
formalism.
