Research Findings: This study compared the effects of content-based shared book-reading instruction versus an explicit vocabulary-only condition on the vocabulary development of preschool dual language learners (DLLs). Using shared book reading as the mode of instruction, we randomly assigned 48 bilingual preschool teachers and 281 Spanish-speaking DLLs from their classrooms at the class level to either a content-enriched interactive book-reading intervention or a thematic content-related vocabulary-only condition. Screened using the preLAS(R), the DLLs were selected for the study based on their scores at the prefunctional and beginning levels of English proficiency. Intervention and comparison bilingual teachers implemented the assigned instructional approach in small groups, for 20 min daily, for 18 weeks. Based on the results of multilevel models, findings indicated pre- to posttest growth on taught words for each instructional condition, with no significant effects on standardized English language measures. Practice or Policy: Results suggest that preschool DLL children benefit from the systematic use of interactive content-enriched shared book-reading vocabulary instruction or direct teaching of content vocabulary when instruction integrates language interaction opportunities with higher cognitive talk and scaffolds. The vocabulary-only routine is a cost-effective approach but may not replace the broader benefits of reading books. (author abstract)