  This paper derives an explicit formula for Branson's Q-curvature in
even-dimensional conformal geometry. The ingredients in the formula come from
the Poincare metric in one higher dimension; hence the formula is called
holographic. When specialized to the conformally flat case, the holographic
formula expresses Q-curvature as a multiple of the Pfaffian and the divergence
of a natural one-form. The paper also outlines the relation between holographic
formulae for Q-curvature and a new theory of conformally covariant families of
differential operators due to the second author.
