In Japanese, numeral classifiers-or measure words-co-occur with numbers in counting phrases. Th e present study characterized parent numeral classifier use and its relation to children’s classifier acquisition and number learning. Twenty-four Japanese-speaking parents and their two- to six-year-old children viewed and talked about two wordless picture books about counting to each other. Children also participated in a Counting task and Give-N task. Results revealed (1) parents’ classifier use changed in relation to children’s age and classifier use, and (2) parents’ increased use of specific classifiers was uniquely associated with children’s number understanding. These results suggest that aspects of children’s language and numerical development are related to parents’ language input, demonstrating the importance of examining the relation between language and cognition in a developmental context.