How young children's and older students' knowledge of words develops--their structure, their meanings, how they work in context--is reflected in the Common Core English Language Arts expectations. Meeting these expectations for each learner requires that we teach in a developmentally-responsive manner. This includes our being familiar with the nature of the English spelling system, determining what each learner knows about the system, and then providing instruction that stretches but does not frustrate learning. There is a reciprocal relationship between reading and spelling words, and understanding how this relationship develops over time is the key to developmentally-responsive decoding and encoding instruction, as well as to developing every learner's vocabulary.