Revision with unchanged content. Requirements Engineering is the Achilles heel of the whole software development process. It involves many stakeholders and includes not only technical but also sociological and psychological activities. Even when all the stakeholders come to a consensus, the produced requirements are rather informal. In the early project phases the functionality of the prospective software is not yet understood in the precision necessary for formalization, which makes requirements formalization not only a refinement, but also a learning process. To facilitate this learning process, this book proposes a novel approach to the extraction of a domain ontology from requirements documents in order to establish a common language for the project stakeholders. An ontology consists of a set of terms and relations between these terms. As compared to a glossary, a domain-specific ontology gives a more explicit definition of terms and relations between them. When the ontology is extracted, a domain expert validates it. The validated ontology becomes both the common language for all the project stakeholders and a valuable resource for later development steps.