This report provides a summary of the completed Overcoming Disadvantage in Early Childhood (ODEC) longitudinal research study. The study was designed to evaluate the Australian Literacy and Numeracy Foundation's (ALNF) Early Language and Literacy (EL&L) program by answering the research question: What is the effect of the EL&L program on the development of language and literacy skills in preschool aged children? An integral component of this study was the development of the Early Language and Literacy Developmental Index (ELLDI), a new tool to measure oral language and literacy learning. The development of the ELLDI was based on the recognised need for a fit for purpose early childhood best practice measure of oral language and literacy. This summary report highlights the demonstrated capacity of the EL&L program to close the language and literacy achievement gap for children who participate in the program, particularly those from disadvantaged backgrounds. It also encapsulates the successful development of the ELLDI and the ELLDI scale to accurately measure and map children's language and literacy development, to inform teaching and learning. [Funding provided by Eureka Benevolent Foundation and CAGES Foundation.]