On many groups, like those conducting IS/IT requirements negotiations or those conducting risk and control selfassessments, decisions cannot be made by decree because team members are co-responsible peers. In such situations, teams must build consensus to gain commitment from all involved. Although consensus has been widely studied, no causal model of the cognitive mechanisms that give rise to consensus has yet emerged. Such a model might be useful for evaluating, for developing effective and efficient strategies for building consensus in a group and for making sense of the models and results in the current consensus literature. In this paper we derive the logic of Consensus Building Theory (CBT). We then illustrate how the mechanisms of the causal theory could be incorporated into a process model of consensus building. We derive a set of diagnostics for discovering the causes of conflict in a group, and we propose strategies based on CBT for attempting resolution of conflicts.