Résumé : This paper studies 3-dimensional visibility representations of graphs in which objects in 3-d correspond to vertices and vertical visibilities between these objects correspond to edges. We ask which classes of simple objects are {\em universal}, i.e. powerful enough to represent all graphs. In particular, we show that there is no constant $k$ for which the class of all polygons having $k$ or fewer sides is universal. However, we show by construction that every graph on $n$ vertices can be represented by polygons each having at most $2n$ sides. The construction can be carried out by an $O(n^2)$ algorithm. We also study the universality of classes of simple objects (translates of a single, not necessarily polygonal object) relative to cliques $K_n$ and similarly relative to complete bipartite graphs $K_{n,m}$.