This chapter describes the nature of high-quality word knowledge and reviews the evidence that it is related to important literacy outcomes, including word reading, spelling, and reading comprehension. High-quality word knowledge goes beyond traditional notions of vocabulary (now thought of as vocabulary breadth) to include orthographic, phonological, semantic, and syntactic information (as argued in Perfetti's, 2007, lexical quality hypothesis), and vocabulary depth (Li & Kirby, 2014). Features of vocabulary depth include multiple meanings, alternative uses, and morphological and etymological knowledge. The role of morphological knowledge is key in this process, morphology acting as a binding agent (Kirby & Bowers, 2017, 2018). The chapter examines the nature of morphological knowledge and how it is related to other language and literacy skills. There is a growing literature indicating that measures of word-knowledge quality predict variance in literacy abilities above and beyond that predicted by vocabulary breadth. There is also a growing literature demonstrating that instruction in high-quality word knowledge, especially in morphological knowledge, leads to better quality problem solving during literacy learning, and improved literacy outcomes. Implications for the improvement of word knowledge and the quality of word knowledge are discussed.