In this chapter, an argumentative approach to ambiguity and commitment attribution is advanced in order to address the problem of establishing the speaker’s commitments in case of ambiguity of his utterance. The goal is to analyze how a doubtful or potentially doubtful interpretation of an utterance can be supported dialectically by providing a dialectical mechanism for establishing which interpretation is the best one. Distinct types of ambiguity are distinguished by pointing out the subtle line between what is directly conveyed and what is left implicit. Our challenge is to develop an argumentative model for representing how it is possible to choose an interpretation over another and justify it in cases of ambiguity. The model that we propose is grounded on the notion of presumption. When an interpretation is challenged, the parties to the discussion need to support their interpretation based on reasons. We conceive these reasons as conclusions of a type of argument called “from best explanation,” which is based on the presumptions available in a given context. An interpretation can be based on various types of evidence leading to an interpretive conclusion through more general or specific presumptions. An interpretation is assessed by evaluating the defeasibility conditions of the presumptions on which it is based.