Do presupposition-triggers just trigger presupposition? In analyzing the semantic and pragmatic constraints on the Japanese particle mo, roughly equivalent to the English too, this book shows how the complex mechanism of the constraints accounts for its discourse function • that is, how it enables the hearer to process the sentence to achieve more effectively the speaker’s intended discourse interpretation. It provides a model to explain how the presupposition of a linguistic form and its discourse function are related to each other. In doing so, it introduces the notion of “contextual relevance”, the relation between a proposition and the context, and models this notion in the case of mo - incorporating the requirement that the proposition of a mo sentence and the context have a common entailment with contextual relevance. The monosemous account of mo also explains how the particle sometimes generates the meaning of even when the context involves scalar expectation. By combining semantic and pragmatic analyses, this book shows how the constraints on the usage of a linguistic form reveal the contribution of the form to the presupposition of the sentence and to its discourse interpretation.