Problem/Purpose: Young children with developmental speech and/or language impairment (DSLI) often fail to develop important oral language and early literacy skills that are foundational for subsequent schooling and reading success. The purpose of this investigation was to examine the efficacy of the TELL curriculum and associated evidence-based teaching practices in promoting the acquisition of oral language and early literacy skills for preschool children with DSLI. Participants: Participants included 202 male and 87 female preschoolers with DSLI in the absence of other developmental impairment. Children ranged in age from 43 to 63 months. They were enrolled in 91 inclusive preschool classes and their corresponding classroom teachers were all female. Method: In this cluster RCT, classroom teachers were randomly assigned to implement the TELL curriculum or to continue with their business-as-usual (BAU) curriculum. Proximal outcomes were assessed with investigator-developed curriculum-based measures (CBM) administered six times over the school year and an investigator-developed assessment of vocabulary targeted in TELL. Standardized tests of oral language ("Clinical Evaluation of Language Fundamentals Preschool - 2nd Edition"), and early literacy skills ("Test of Preschool Early Literacy"), and a benchmarked early literacy assessment ("Phonological Awareness and Literacy Screening PreK") were administered at the beginning and end of the school year to determine impact on more distal outcomes. Results: Results indicated a significant TELL effect for all CBMs at later measurement points with Cohen's "ds" in the medium (0.43) to very large (1.25) range. TELL effects were also noted for the vocabulary measures with small to medium between-group effect sizes (Cohen's f^2 range from 0.02 to 0.44). There were no significant TELL effects for the more distal measures. Conclusion: Based on progress measures, the TELL curriculum was effective for improving the oral language and early literacy skills of young children with DSLI. [This paper will be published in "Early Childhood Research Quarterly."]