Fake supergravity and domain wall stability

We review the generalized Witten-Nester spinor stability argument for flat domain wall solutions of gravitational theories. Neither the field theory nor the solution need be supersymmetric. Nor is the space-time dimension restricted. We develop the nontrivial extension required for AdS-sliced domain walls and apply this to show that the recently proposed ``Janus'' solution of type-IIB supergravity is stable nonperturbatively for a broad class of deformations. Generalizations of this solution to arbitrary dimension and a simple curious linear dilaton solution of type-IIB supergravity are by-products of this work.