There is a need for empirically based educational practices shown to support learning, yet validation tends to require a high degree of experimental control that can limit ecological validity and translation to classrooms. We describe our iterative intervention design to support preschoolers' vocabulary through book reading coupled with playful learning, including the process of translating research-based methods to an authentic teacher-delivered intervention. Effectiveness of the teacher-implemented intervention was examined by comparing book reading alone versus book reading plus play in supporting vocabulary development in preschoolers (N = 227) from low-income families with diverse backgrounds. Teachers used definitions, gestures, and pictures to teach vocabulary. During play, teachers led play with story-related figurines while using target vocabulary. Ten teachers read books and engaged children in play (read + play [R + P]), and 6 used only book reading (read-only [RO]). For children in both the R + P and RO conditions, within-subjects analyses of gains on taught versus control words revealed large effects on receptive (R + P, d = 1.08; RO, d = 0.92) and expressive vocabulary (R + P, d = 1.41; RO, d =1.23). Read-only had a statistically significant effect (d = 0.20) on a standardized measure of receptive vocabulary, but there were no statistically significant differences between conditions. Moderate to large effects were found using an expressive task when words were tested 4 months after they were taught. Implications for curriculum design and the potential benefits of enhancing children's vocabulary through book reading and playful learning are discussed.