Abstract. The Delaunay tessellation of a locally finite subset of the hyperbolic space H n is constructed via convex hulls in R n+1 . For finite and lattice-invariant sets it is proven to be a polyhedral decomposition, and versions (necessarily modified from the Euclidean setting) of the empty circumspheres condition and geometric duality with the Voronoi tessellation are proved. Some pathological examples of infinite, non lattice-invariant sets are exhibited.The main theorem of this paper describes a "convex hull construction" of canonical polyhedral decompositions with prescribed vertex set for arbitrary complete, finite-volume hyperbolic manifolds and locally finite subsets. Various versions of this construction have been useful in the study of hyperbolic manifolds, beginning with work of Epstein-Penner in which the "vertex set" is essentially a collection of horospherical cusp neighborhoods [13]. Charney-Davis-Moussong gave a version for finite subsets of closed hyperbolic manifolds [7], generalizing earlier work of Näätänen-Penner [15]. More recently, Cooper-Long translated the Epstein-Penner construction to the setting of convex projective manifolds [8].Our results are complementary to [13] and generalize the main case of [7]. The main new case here, which is an important tool in our subsequent works [11] and [10], covers finite subsets of finite-volume non-compact hyperbolic manifolds. Compared to previous work this case exhibits substantial differences in the nature of the cells produced and of the decomposition's "geometric duality" relationship with the Voronoi tessellation. As we will describe below the statement, the source of these differences also significantly complicates the proof. We call our decomposition the "Delaunay tessellation" because it is characterized by an empty circumspheres condition, property (2) below. It is constructed in Definition 3.1.Theorem 6.23. Let Γ < SO + (1, n) be a torsion-free lattice and S a non-empty, locally finite, Γ-invariant set in H n . The Delaunay tessellation of S is a locally finite, Γ-invariant collection of convex polyhedra (the cells) whose union is H n , satisfying:(1) Each face of each cell is a cell, and distinct cells that intersect do so in a face of each; i.e. it is a polyhedral complex in the sense of eg. [9, Dfn. 2.1.5], with vertex set S. (2) For each metric ball or horoball B of H n that intersects S but only on its boundary, ie. such that S = ∂B satisfies B ∩ S = S ∩ S, the closed convex hull of S ∩ S in H n is a Delaunay cell contained in B. Each Delaunay cell has this form. (3) For each parabolic fixed point U of Γ such that there is a horoball centered at U and disjoint from S, there is a unique horosphere S centered at U such that the closed convex hull of S ∩ S in H n is a Γ U -invariant n-cell, where Γ U is the stabilizer of U in Γ. Each other cell is compact and has a metric circumsphere. The Delaunay tessellation is uniquely determined by condition (2) above.Here and in the remainder of the paper, we use the hyperboloid model of hyperbo...