This paper presents a new approach for linking Requirements Engineering activities into a Process Framework that can br used as a reference for driving concrete Requirements Engineering processes. Special emphasis is placed on constructing Problem Domain models in order to build a common understanding of the problem context, to situate user requirements with reference to it, and as a technique for user requirements refinement. Another aspect highlighted by the Process Framework is the separation of the elicitation, analysis and refinement of user requirements from the construction of a system or software specification. Although the emphasis is placed on the process and the activities to be performed, the paper describes specific techniques that have been used successfully. The Process Framework is supported by a tool and has been tested in several projects from different organizations. This paper describes these experiences, draws conclusions and highlights future issues.