This article describes a model of literacy development as reflected in students' spelling. The model, based on research that identified five stages of word knowledge, explains the development of this knowledge in readers and writers, and provides a framework for elementary-grade instruction that is intended to: (1) address grade-level expectations in the "Common Core State Standards, English Language Arts"; and (2) describe a developmentally appropriate approach to instruction. The model also informs the assessment of word or orthographic knowledge to gain insight into the range of developmental levels in a class and to guide instruction in whole-class and small-group contexts. Examples focus on a third-grade classroom in which the range of levels includes beginning, transitional, and intermediate readers and writers, and suggest implications of developmentally grounded instruction for students' growth toward achieving grade-specific "CCSS/ELA."