Estimating native-speaker vocabulary size is important for guiding interventions to support native-speaker vocabulary growth and for setting goals for learners of English as a foreign language. Unfortunately, the measurement of native-speaker vocabulary size has been one of the most methodologically contentious areas of research in applied linguistics, with estimates of adults’ vocabulary size ranging from 12,000 words to well over 200,000 words. This book reviews over one hundred years of research, critically examining the methodological issues and findings at each age level from young children to adults, and suggesting solutions. It presents a model organising the factors involved in vocabulary growth and is rich in well-researched suggestions for supporting native-speaker vocabulary learning. It concludes with topics for further research. The research shows that we now have a more stable and coherent picture of what and how much vocabulary native-speakers know, and how this knowledge grows throughout their lives.