A study was conducted in 26 Head Start classrooms with 264 children to compare the effect of a read aloud plus extension activities intervention over a control group to the effect of a read aloud only intervention over a control group on preschool children's vocabulary. Children were assessed before and after the intervention on target vocabulary and general vocabulary measures. Research Findings: The results suggest that the effects of the read aloud plus intervention were stronger than the effects of the read aloud only intervention on target word learning. In addition, the effects of the read aloud plus intervention on target word learning were stronger for children with higher versus lower general vocabulary knowledge. Neither intervention had an effect on general word knowledge. Practice or Policy: Observation and fidelity data are used to contextualize the findings, and the results are discussed in light of the extant literature on preschool vocabulary interventions. (Contains 8 tables.)