ESPRIT project NATURE aims at the development of Novel Approaches to Theories Underlying Requirements Engineering. Such approaches are necessary because the environment in which RE has to operate has changed dramatically since the current methods were invented. The task of RE has moved from supporting the early phases of individual projects, to accompanying the whole lifecycle of complex, long-lived human-machine systems in a rapidly changing organizational environment. The NATURE framework addresses these new demands by defining a novel framwork based on the idea that requirements engineering is a continous process of establishing visions of different stakeholders in a complex context. Around this framework, NATURE has developed three specific theories. The requirements domain theory gives advice what context knowledge is relevant and how to organize it. The requirements process theory offers a unified process meta model in which more flexibility than other software process or workflow models. The knowledge representation theory aims at defining what domain and process knowledge to capture, and how to manage this knowledge using an effective mix of informal, semi-formal and formal representations. While companion papers in this volume address the domain and process theories, we first present the overall NATURE framework and its implications for research directions in RE, and then focus on the knowledge representation theory. Specifically, we discuss the implications of the framework for repository-based process-centered RE environments, and some extensions towards large-scale distributed requirements management based on a generalization of the viewpoints idea. The concepts have been validated in a comprehensive prototype environment called PRO-ART which is also presented.