This article is a commentary on a model for negotiation in teaching-learning dialogues (Baker 1994) that traces its origins and developments over the past 20 years. The first main section of the paper describes the research background out of which the model arose, within the "credo" of individualised tutoring of the 1980s. This is followed by a summary of the main elements of the model, then a presentation of its subsequent developments. These comprise the analysis of argumentation dialogue and its potential for collaborative learning, the analysis of interpersonal relations in relation to the interactive regulation of affect, and extensions of the model to other epistemic situations such as explanation generation, co-design, online epistemic discussions and use of communication interfaces in computer-supported collaborative learning environments. On these bases, the object of study of this research is defined as the processes of negotiation of meaning in epistemic situations. It is concluded that the main core of the model was retained, throughout its deepening and extension, but the underlying theory was radically changed, from cognitivist belief-systems to a view of dialogue itself as collective thinking. Two challenges for artificial intelligence and education research are raised: the formalisation of interpersonal relations as they are played out in social interaction, and the analysis of the processes of appropriation of discourse genres.