  This paper is devoted to the study of couplings of the Lebesgue measure and
the Poisson point process. We prove existence and uniqueness of an optimal
coupling whenever the asymptotic mean transportation cost is finite. Moreover,
we give precise conditions for the latter which demonstrate a sharp threshold
at d=2. The cost will be defined in terms of an arbitrary increasing function
of the distance. The coupling will be realized by means of a transport map
("allocation map") which assigns to each Poisson point a set ("cell") of
Lebesgue measure 1. In the case of quadratic costs, all these cells will be
convex polytopes.
