We review the relations between a family of domain-wall solutions to M-theory
and gravitational instantons with special holonomy. When oxidized into the
maximal-dimension parent supergravity, the transverse spaces of these domain
walls become cohomogeneity-one spaces with generalized Heisenberg symmetries
and a homothetic conformal symmetry. These metrics may also be obtained as
scaling limits of generalized Eguchi-Hanson metrics, or, with appropriate
discrete identifications, from generalized Atiyah-Hitchin metrics, thus
providing field-theoretic realizations of string-theory orientifolds.