This chapter reviews the connections between number marking (specifically, singular and plural marking) and the mass–count distinction. It explores how different semantic theories of number marking interact with various ontological theories of the mass–count distinction. It also discusses a growing tension within the mass–count literature. On one hand, there are many semantic and syntactic similarities between mass nouns and plurals, which suggests that the two subcategories might have many features in common. On the other hand, verbal, auxiliary, and determiner agreement patterns suggest that mass nouns share certain syntactic properties with singular count nouns. Yet, singular count nouns and plural count nouns hardly share any properties in common, both in terms of their syntactic distribution and their semantic implications. The chapter discusses two potential resolutions to this growing tension.