  In this paper, we show the existence of (co-oriented) contact structures on
certain classes of $G_2$-manifolds, and that these two structures are
compatible in certain ways. Moreover, we prove that any seven-manifold with a
spin structure (and so any manifold with $G_2$-structure) admits an almost
contact structure. We also construct explicit almost contact metric structures
on manifolds with $G_2$-structures.
