This talk will describe an analogue of a Dirichlet to Neumann
map for Poincar\'e-Einstein metrics, also known as asymptotically
hyperbolic or conformally compact Einstein metrics. An explicit
identification of the linearization of the map at the sphere will be
given for even interior dimensions, together with applications
to the structure of the map near the sphere and to a different proof of
the positive frequency conjecture of LeBrun which was resolved by Biquard.