Past research has indicated that parent-child book reading interaction quality, not just quantity, is a relevant consideration in children's vocabulary acquisition. In particular, children's active participation is considered key. This study investigated links between maternal beliefs about shared reading strategies and children's readiness to learn to read, mothers' observed shared reading behaviors, and children's vocabulary sizes. Participants were 62 mothers and their preschool children attending Head Start. Mother-child book reading was observed, and mothers' beliefs were measured via self-report and ratings of videotaped vignettes portraying reading strategies. Children's expressive and receptive vocabularies were assessed. Results revealed that dyadic shared reading quality mediated the link between mothers' beliefs and children's expressive, but not receptive, vocabulary. Further, mothers' perceptions of children's readiness to learn to read moderated the link between mother's beliefs and shared reading quality. This evidence highlights the importance of maternal beliefs in guiding behaviors that facilitate preschoolers' vocabulary development. (author abstract)