  In this survey, we describe invariants that can be used to distinguish
connected components of the moduli space of holonomy G_2 metrics on a closed
7-manifold, or to distinguish G_2-manifolds that are homeomorphic but not
diffeomorphic. We also describe the twisted connected sum and extra-twisted
connected sum constructions used to realise G_2-manifolds for which the above
invariants differ.
