Children coming from disadvantaged backgrounds tend to have smaller vocabularies than their higher income peers, which can lead to further academic discrepancies over the early school years. This suggests the need for intervention research that addresses various ways to boost low-income children's word knowledge before the start of kindergarten. This study examines how different forms of scaffolded adult extra-textual input (questions, comments, or a combination of the two) during shared book reading might enhance vocabulary acquisition in young children attending Head Start. Additionally, child characteristics such as general vocabulary level and executive functioning are examined as potential predictors for vocabulary learning in the context of shared book reading. Results indicated that children who heard extra-textual input during shared book reading were able to receptively identify more new vocabulary words than those who did not hear any extra-textual input. No significant differences were found between groups that heard scaffolded questions, comments, a combination, or simple repetition of the target words, however. Results also indicated that children's general vocabulary level was a positive predictor of their new vocabulary word learning during shared book reading after controlling for age, and children's executive functioning skills moderated the effect of extra-textual input vs. no extra-textual input on children's expressive and definitional vocabulary learning. Implications for early literacy intervention and Head Start classroom practices are discussed.