Children with the lowest oral language and early literacy skills at entry to kindergarten are the most vulnerable to poor literacy outcomes. This article describes the programmatic development of a Tier 3 early literacy intervention for preschool children who are most in need of intensive support to achieve early literacy outcomes. The intervention consists of carefully sequenced activities and games to promote early literacy development and is designed to be implemented by an early childhood educator with small groups of one to two children during center time or small-group instruction time in the classroom. Development of, and research on, the intervention followed an iterative process leading to a sequence of studies demonstrating moderate to strong effect sizes for children who qualified for Tier 3 support. Children who received intervention were a diverse group who presented an array of challenging characteristics that required individualization of the intervention. Children progressed at different rates, and some children clearly needed more opportunities to receive the intervention. Further research is needed to investigate factors related to effectiveness of Tier 3 interventions, including characteristics of the intervention, such as duration, time, and group size, as well as the characteristics of children who respond/do not respond. (author abstract)