A model of requirements determination as the process oi establishing visions in context explains how both new ideas and existing habits influence diversity in a family of information systems applications. Visions are operationalized as non-functional requirements that are broken down according to constraints imposed by context and traded off against other non-functional requirements. Context is organized according to four "worlds," taking into account the need for considering application domain (subject world), organizational context (usage world), existing systems (system world), and the development environment itself (development world). Process is modeled as driven by context-dependent decisions that, together with external factors, cause moves within a three-dimensional space of cognitive understanding, social agreement, and technical representation. The framework leads to a formally based and computer supported requirements engineering environment that is currently developed and practically evaluated by the ESPRIT project NATURE.