Vocabulary richness, including lexical diversity and use of rare words, has an important role in assessing proficiency, diagnosing progress and testing theory in the study of language development. This book reviews different methods for quantifying how vocabulary is deployed in spontaneous speech and writing, before introducing an alternative approach which can assess overall lexical diversity, measure morphology development and compare the development of different word classes. The new approach is illustrated by its application to first and second language learners.