For large and complex design-processes traditional approaches to process design may no longer suffice. Prescriptive knowledge models may support a more professional approach to process design. However, their impact on the practice of process design is still too limited. The reasons for this may include that the potential of professional process design to produce effective and efficient design processes is still underestimated as well as the potential of prescriptive design knowledge to support that professional process design. This paper contributes to a better understanding of these potentials by analysing the nature of prescriptive design knowledge, the way it can be used to produce sound design processes and an approach to develop prescriptive process-design knowledge. It describes prescriptive design knowledge in terms of field-tested and grounded technological rules and the impact of their application on actual design processes through a dual redesign process.