To produce effective advice several sources of knowledge are needed. Knowledge about the application domain the advice is concerned with is of course necessary, but not sufficient. If the aim of the intervention is inducing people to modify their habits, we also need specific theories of how and why people change behaviour to guide the advising process. In some cases, however, it still does not suffice: when suggesting a change in a well established habit, several factors have to be taken into account, and a good adviser might also need argumentative capabilities, in order to overcome possible personal and environmental barriers to the change. This paper presents a model of advice giving that integrates Artificial Intelligence with concepts and methods coming from different disciplines. The model has been implemented in Daphne, an advice giving system that operates in the nutrition education domain.