Young children with developmental speech and/or language impairment (DSLI) often fail to develop 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 Teaching Early Literacy and Language (TELL) curriculum and associated evidence-based teaching practices. Participants included 91 preschool classroom teachers and 202 male and 87 female preschoolers with DSLI who were enrolled in their classes. Children ranged in age from 43 to 63 months. 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 (CBMs) administered six times over the school year. Distal tests (pre-post) of oral language and early literacy skills included an investigator-developed pre-post expressive and receptive vocabulary test, two additional standardized measures (Clinical Evaluation of Language Fundamentals-Preschool 2nd Edition, the Test of Preschool Early Literacy). A benchmarked early literacy assessment, the Phonological Awareness and Literacy Screening PreK, was also administered. 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 distal vocabulary measure 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 standardized distal measures. Based on progress measures, the TELL curriculum was effective for improving the oral language and early literacy skills of young children with DSLI. (author abstract)