Abstract. We generalize the relative (co)tilting theory of Auslander-Solberg in the category mod Λ of finitely generated left modules over an artin algebra Λ to certain subcategories of mod Λ. We then use the theory (relative (co)tilting theory in subcategories) to generalize one of the main result of Marcos et al. [Comm. Algebra 33 (2005)].Introduction. Let Λ be an artin algebra, and let mod Λ denote the category of finitely generated left Λ-modules. Auslander and Solberg [9, 10] developed a relative (co)tilting theory in mod Λ which is a generalization of standard (co)tilting theory [3], [12], [14], [23]. In this paper we develop a relative (co)tilting theory in extension-closed functorially finite subcategories of mod Λ.Let T be an ordinary tilting module over Λ. Then the module DT , where D is the usual duality between left and right modules, is a cotilting module