We state that any constant curvature Riemannian metric with conical
singularities of constant sign curvature on a compact (orientable) surface S
can be realized as a convex polyhedron in a Riemannian or Lorentzian)
space-form. Moreover such a polyhedron is unique, up to global isometries,
among convex polyhedra invariant under isometries acting on a totally umbilical
surface. This general statement falls apart into 10 different cases. The cases
when S is the sphere are classical.Comment: Survey paper. No proof. 10 page