It is a great pleasure to see in print this paper by Knight and Stiny (arq 5/4, pp 355–372), who have been collaborating for many years, Stiny predominantly as a theoretician, and Knight as a design teacher. Despite their close association, they have rarely published together, and it was something of a landmark when they gave a joint keynote speech, received with great warmth, to the conference in Greenwich in January 2000. The field in which they engage has grown rapidly, and few can be better placed to review its achievements, and establish a basic taxonomy to classify existing work, and map areas for further research.