Let R be a ring, see below for other notation. The functor categories (mod-/?,Ab) and ((/?-mod) op , Ab) have received considerable attention since the 1960s. The first of these has achieved prominence in the model theory of modules and most particularly in the investigation of the representation theory of Artinian algebras. Both [11, Chapter 12] and [8] contain accounts of the use (mod-/?, Ab) may be put to in the model theoretic setting, and Auslander's review, [1], details the application of (mod-/?, Ab) to the study of Artinian algebras. The category ((/?-mod) op , Ab) has been less fully exploited. Much work, however, has been devoted to the study of the transpose functor between 7?-mod and mod-/?. Warfield's paper, [13], describes this for semiperfect rings, and this duality is an essential component in the construction of almost split sequences over Artinian algebras, see [4]. In comparison, the general case has been neglected. This paper seeks to remedy this situation, giving a concrete description of the resulting equivalence between (mod-/?, Ab) and ((/?-mod) op , Ab) for an arbitrary ring /?.The first two sections are concerned with detailing an equivalence between ((/?-mod ) op , Ab) and (mod-/?, Ab). The main result, Theorem 2.5, states that the equivalence between these categories may be built from the functors Tor(-,-) and Ext(-,-). Parts of this theorem are well known. For instance the functor Tor(-,-) has been extensively studied in this setting when R is an Artinian algebra, see [3]. In the third section this equivalence is applied to the Ziegler spectra of /?. In Theorem 3.3, it is shown that the sets {M | fp-inj.dimM < n] and {jV|w.dinr/V < n\,n a natural number, are closed in their respective spectra when /? is left coherent. Furthermore, it is also demonstrated that these sets are mapped onto each other under Herzog's correspondence between the closed sets of the Ziegler spectra, [7, Theorem 5.5]. The main result of Section 3, Corollary 3.4, states that when R is left coherent both spectra are test spaces for right finititistic weak dimension of /?. Two special cases are noted: when R is left Noetherian and when /? is left coherent and right perfect.Throughout R denotes an associative ring with unity. Mod-/? is the category of right /?-modules, mod-/? the category of finitely presented right /?-modules, mod-/? is the quotient category of mod-/? modulo the ideal of those maps that factor through a projective. The left handed analogues of these categories are written as /?-Mod, /?-mod and /?-mod, respectively. If C is a small additive category then (C, Ab) (respectively (C op , Ab)) is the category of covariant (respectively contravariant) additive functors from C to the category of abelian groups; (C, Ab) fp is the category of finitely presented objects of (C, Ab). If