This project began when one of my research colleagues, a civil engineer, pointed to his very thorough study of daylighting in buildings and said: ‘Here is all that architects need to know about light – why don't they use it?’ A discussion followed about what kind of knowledge is relevant, when, and for whom. It ended with the questions: ‘What kind of knowledge do architects really use in their design process?’ and ‘How can we as researchers provide the different kinds of knowledge that are useful for practice?’ These questions became the basis of a research project and this paper reports on its studies and discussions.