Injective metric spaces, or absolute 1-Lipschitz retracts, share a number of properties with CAT(0) spaces. In the '60s Isbell showed that every metric space X has an injective hull E(X). Here it is proved that if X is the vertex set of a connected locally finite graph with a uniform stability property of intervals, then E(X) is a locally finite polyhedral complex with finitely many isometry types of n-cells, isometric to polytopes in l n ∞ , for each n. This applies to a class of finitely generated groups Γ, including all word hyperbolic groups and abelian groups, among others. Then Γ acts properly on E(Γ) by cellular isometries, and the first barycentric subdivision of E(Γ) is a model for the classifying space EΓ for proper actions. If Γ is hyperbolic, E(Γ) is finite dimensional and the action is cocompact. In particular, every hyperbolic group acts properly and cocompactly on a space of non-positive curvature in a weak (but non-coarse) sense.finite then the injective hull is a finite polyhedral complex of dimension at most 1 2 |X| whose n-cells are isometric to polytopes in l n ∞ = l ∞ ({1, . . . , n}). A detailed account of injective metric spaces and hulls is given below, in Secs. 2 and 3.Isbell's construction was rediscovered 20 years later by Dress [13] (and even another time in [10]). Due to this independent work and a characterization of injective metric spaces from [3], metric injective hulls are also called tight spans or hyperconvex hulls in the literature, furthermore "hull" is often substituted by "envelope". Tight spans are widely known in discrete mathematics and have notably been used in phylogenetic analysis (see [17,16] for some surveys). Apart from the two foundational papers [24,13] and some work referring to Banach spaces (see, for instance, [25,36,11]), the vast literature on metric injective hulls deals almost exclusively with finite metric spaces. Dress proved that for certain discrete metric spaces X the tight span T X still has a polyhedral structure (see Items (5.19), (6.2), and (6.6) in [13]); these results, however, presuppose that T X is locally finite dimensional. A simple sufficient, geometric condition on X to this effect has been missing (but see Theorem 9 and Item (5.14) in [13]).Here it is now shown that, in the case of integer valued metrics, a weak form of the fellow traveler property for discrete geodesics serves the purpose and even ensures that E(X) is proper, provided X is; see Theorem 1.1 below. The polyhedral structure of E(X) and the possible isometry types of cells are described in detail and no prior knowledge of the constructions in [24, 13] is assumed. With regard to applications in geometric group theory, a general fixed point theorem for injective metric spaces is pointed out (Proposition 1.2), which closely parallels the wellknown result for CAT(0) spaces. It has been known for some time that if the metric space X is δ-hyperbolic, then so is E(X), and this implies that E(X) is within finite distance of e(X), provided X is geodesic or discretely geodesic (Propos...