Issues related to the estimation of individuals' vocabulary size are discussed, including the rationale for vocabulary size research and the psychological, pedagogical, and quantitative approaches to vocabulary research and methodological problems associated with them. Some results from a large-scale assessment of Finnish comprehensive school students' active and passive vocabularies, word-formation skills, and contextual inference abilities in English are outlined. Resulting vocabulary research directions are suggested in two major areas: test types and student populations. It is recommended that research on test types focus on how to tap partial knowledge of word meanings and their effect on vocabulary size estimates and on estimation of vocabulary in the context of discourse comprehension and production. It is also suggested that the student populations studied be extended to include lower stages of vocabulary development, end-of-secondary school and university students, students with more training in word analysis and context utilization, and students at different ability levels. In addition, theoretical inquiry on the nature of vocabulary learning, teaching, and research is recommended. (MSE)