According to Markman and Wachtel, children assume that nouns pick out mutually exclusive object categories, and so each object should have only one category label. While this assumption can be useful in word learning, it is not entirely reliable. Therefore, children need to learn when to and when not to make this assumption. 6 studies examined whether knowledge about hierarchical organization of categories and about cross-language equivalents for object labels can help children limit their use of this assumption appropriately. These studies revealed that adults as well as children resisted assigning 2 novel names to the same object in some situations. By age 4, children also seemed to know enough about categorization to accept 2 names for an object if the names picked out categories from different levels of a hierarchy (e.g., animal and lemur) but not if they picked out categories from the same level (e.g., lemur and seal). Moreover, monolingual as well as bilingual children seemed to know enough about languages to accept 2 names for the same object if the names clearly came from different languages. Together, these findings suggest that even preschool children can make use of knowledge about language and categorization to fine tune the mutual exclusivity assumption in order to use it effectively in word learning.