Reading difficulties continue to be a pervasive public health issue. Many children experience reading difficulties due to underdeveloped phonological awareness – and, specifically, phonemic awareness. Although decades of basic research have established phonemic awareness as causally related to reading acquisition as well as a developmental progression from phonological sensitivity to phonemic awareness, we know much less about how or when to support phonological awareness development in children. Our long-term goal is to optimize phonological awareness intervention as a means of preventing reading difficulties. Our short-term goal is to complete two randomized controlled trials that inform such optimization through addressing current scientific controversies around the content, timing, and goals of phonological awareness intervention. In the randomized controlled trials, we evaluate three interventions (phonological sensitivity + phonemic awareness, phonemic awareness only, delayed phonemic awareness), which vary in the linguistic units targeted but are carefully equated on content and/or instructional time, for their impact on children’s phonemic awareness, reading, and spelling skills relative to each other and to a control condition; we also consider whether impacts differ based on when and to whom intervention is provided. Participating preschool and kindergarten children contribute screening, pretest, midtest, posttest, and longitudinal data to address four aims: (Specific Aim 1) Determine the relative impact on phonemic awareness, and subsequent reading and spelling development, of intervention that (a) initially targets phonological sensitivity or (b) directly targets only phonemic awareness, (Specific Aim 2) Determine the relative efficacy of these two approaches when intervening during preschool or kindergarten, (Secondary Aim 3) Examine whether the impact of the two approaches is differential based on children’s initial phonological skills and alphabet knowledge as well as sociodemographic factors, and (Secondary Aim 4) Explore whether a transition point can be identified as to when phonemic awareness intervention is most likely to be efficacious. The results will contribute to both basic science concerning the development of phonological awareness and its contribution to reading and spelling acquisition as well as translational efforts. With respect to the latter, results will inform learning standards, instructional recommendations, and intervention design to better align these with the scientific evidence base and thereby improve prevention and intervention for reading difficulties.