A better understanding of the nature of consultations between professionals engaging in the collaborative process of solving complex problems — expertise in use — offers the potential to reshape our ideas about how to design computer systems that can engage in collaborative problem solving with their human cohorts. The research reported here has sought to account for key behaviors contributing to successful consultation, as identified by a cognitive task assessment of human-human consultation discourse in the medical teaching rounds setting. W e have come to view the communication acts of the presenter/investigator as evidence of his deliberate intention to indirectly construct a particular model of the patient's case — his model — in the expert's mind, resulting in two separate but related diagnostic tasks for the expert: one at the patient level and one at the presenter/investigator level. This dual-diagnostic theory of expert understanding of the presenter/investigator's communication actions is partially implemented in the RUMINATE program. The theory provides insights into the expert's capacity to model aspects of the presenter/investigator's competence — insights that contribute to our understanding of expertise embedded in the context of collaborative problem solving discourse.