Developing a large and rich vocabulary is central to learning to read. Children must know the words that make up written texts in order to understand them, especially as the vocabulary demands of content-related materials increase in the upper grades. Studies have documented that the size of a person's vocabulary is strongly related to how well that person understands what he or she reads, not only in the primary grades, but in high school as well. There are profound differences in vocabulary knowledge among young learners from different socioeconomic groups. Many children from high- poverty circumstances will have had fewer experiences with the academic language than the standards require. Children who enter school in these situations will need skillfully developed instruction that not only improves their word knowledge and concepts, but actually accelerates their vocabulary development, maximizing the limited time they have in school. The purpose of this article is to explain the authors' rationale for content-rich oral vocabulary instruction in the age of the Common Core State Standards, and how to effectively build children's vocabulary. First, they dispel some of the common myths about oral vocabulary development, which have often led to a lack of attention for this important topic in school instruction. The authors then move on to a set of instructional principles that should guide teachers' work. Included in this article is a sidebar, by the same authors, entitled "Evidence of Student Learning."