This chapter demonstrates the use of the Distributed Description Logics framework (DDL) and the distributed reasoner DRAGO as formal and practical tools for composing modular ontologies from purely terminological
ontology modules. According to DDL vision, a modular ontology can be formally represented by a distributed T-box, comprising a set of separate T-boxes (one for each ontological module), which are pairwise interrelated by “bridge rules” (inter-module connectives allowing to access and import knowledge contained in modules). The chapter gives the semantic explanations of knowledge import via bridge rules as well as presents the distributed tableaux reasoning technique for its computation. Practically, the implementation of the distributed tableaux in DRAGO reasoner and its use for modular ontology composition is described and experimentally evaluated.