In this paper, we present a framework for developing ontologies in a modular manner, which is based on the notions of interfaces and knowledge encapsulation. Within the context of this framework, an ontology can be defined and developed as a set of ontology modules that can access the knowledge bases of the others through their well-defined interfaces. An important implication of the proposed framework is that ontology modules can be developed completely independent of each others’ signature and language. Such modules are free to only utilize the required knowledge segments of the others. We describe the interface-based modular ontology formalism, which theoretically supports this framework and present its distinctive features compared to the exiting modular ontology formalisms. We also describe the real-world design and implementation of the framework for creating modular ontologies by extending OWL-DL and modifying the Swoop interfaces and reasoners.