Reuse, system integration, and interoperability create a growing need for capturing, representing, and using application-level information about software-intensive systems and their evolution. In ESPRIT Basic Research Project NATURE, we are developing an integrative approach to requirements management based on a three-dimensional framework which addresses formalism as well as cognitive and social aspects. This leads to a new requirements process model which integrates human freedoms through allowing relatively free decisions in given situations. Classes of situations and decisions are defined with respect to the three-dimensional framework through the integration of informal and formal representations, theories of domain modeling, and the explicit consideration of nonfunctional requirements in teamwork. Technical support is provided by a conceptual modeling environment with knowledge acquisition through interactive as well as reverse modeling, and with similarity-based querying.