This book traces the development of a personal research programme over a period of many years. The starting point for the programme was a realisation that research in design seemed to have no clear goal of what it was trying to achieve. A key insight for me was to realise that if we wanted to develop a robust, independent discipline of design (rather than let design be subsumed within paradigms of science or the arts), then we had to be much more articulate about the particular nature of design activity, design behaviour and design cognition. We had to build a network of arguments and evidence for ‘designerly ways of knowing’. The research programme has included some empirical, laboratory-based work, but has also included theoretical reflection, and attempts to review and synthesise the work of other researchers. I have reported this work at various times and in various places – in lectures, conference presentations and journal papers. In this book I have brought together a selected series of these reports, trying to trace a coherent thread, and to lay out some of the network of arguments and evidence referred to above. My goal has been to understand how designers think, or the nature of design expertise, trying to establish its particular strengths and weaknesses, and giving credit where it might be due for design cognition as an essential aspect of human intelligence.