The Aries environment, which solves many of the challenges of applying knowledge-based methods to the development of software requirements and specifications, is described. Using Aries, analysts can develop detailed, precise, validated system specifications without getting bogged down in mundane details. They can also avoid considerable formal syntax and compose solutions in an order-independent way. Aries modularized central knowledge base uses a common underlying representation, enabling analysts to reuse and share various types of knowledge. Its presentation facility lets analysts encode and view a system description using various notations, and focus on specific aspects of the system being described. Its analysis tools help validate even incomplete system descriptions, and a library of transformations lets analysts elaborate and modify system descriptions without introducing inadvertent errors.