This monograph is a synthesis of research findings of the National Institute of Child Health and Human Development research program concerning how children learn to read, why some children and adults have difficulties learning to read, and effective ways to help children learn to read. The review covered studies performed over the last 33 years at 41 research sites throughout the world with more than 34,500 children and adults. Evidence and findings are outlined for each of these three areas. Among nine conclusions concerning how children learn to read is that reading is not a natural process and learning to read is a lengthy process that begins substantially before formal schooling. The 15 conclusions for why some individuals have difficulties learning to read are grouped into those concerned with prevalence, developmental course, and psychometric characteristics and those concerned with environmental, experiential, and individual difference factors. In presenting conclusions about reading instruction, the seven conclusions focus on which teaching approaches and strategies are most beneficial for which children at which stages of reading development. (DB)