have developed a module theory for vertex operator algebras that endows suitably chosen module categories with the structure of braided monoidal categories. Included in the theory is a functor which assigns to discretely strongly graded modules a contragredient module, obtained as a gradewise dual. In this paper, we show that this gradewise dual endows the module category with the structure of a ribbon Grothendieck-Verdier category. This duality structure is more general than that of a rigid monoidal category; in contrast to rigidity, it naturally accommodates the fact that a vertex operator algebra and its gradewise dual need not be isomorphic as modules and that the tensor product of modules over vertex operator algebras need not be exact.We develop criteria which allow the detection of ribbon Grothendieck-Verdier equivalences and use them to explore ribbon Grothendieck-Verdier structures in the example of the rank n Heisenberg vertex operator algebra or chiral free boson on a not necessarily full rank even lattice with arbitrary choice of conformal vector. We show that these categories are equivalent, as ribbon Grothendieck-Verdier categories, to certain categories of graded vector spaces and categories of modules over a certain Hopf algebra. R ALLEN, S LENTNER, C SCHWEIGERT, AND S WOOD where D is a contravariant equivalence of categories. In the context of vertex operator algebra module categories the dualising object K should be thought of as the gradewise dual of the vertex operator algebra, seen as a module over itself, and D as the functor which assigns to any object its gradewise dual and to any morphism its transpose. This seemingly simple definition of a Grothendieck-Verdier category has important consequences, for example, it guarantees the existence of inner Homs for all objects of C and implies that the tensor product of C is right exact, if the category is abelian.Intriguingly, every Grothendieck-Verdier category is also endowed with a second tensor product X • Y = D −1 (DY ⊗ DX) which turns out to be left exact [10,11], again, if the category is abelian. The two tensor products • and ⊗ should be considered on an equal footing. It remains to be discovered what the full implication of these two tensor products is for vertex operator algebras and conformal field theories. Rigid categories are examples of Grothendieck-Verdier categories, where the tensor unit is a dualising object, though the tensor unit being a dualising object does not imply that the category is rigid in general.The notion of a Grothendieck-Verdier category (no rigidity assumed) is nicely compatible with additional structure on the category C , for example, a braiding, and it is possible to introduce notions of a balancing and a twist. It is thus not surprising that this structure has surfaced in numerous disparate places: Grothendieck-Verdier categories are also known as * -autonomous categories [12], however, in this paper, we use the more recent terminology of Grothendieck-Verdier categories [10,11,13]. The main insight of this p...