This report reviews what is known about the role of vocabulary knowledge or knowledge of word meanings in reading comprehension. It states that while an assessment of the number of meanings a reader knows enables a remarkably accurate prediction of an individual's ability to comprehend discourse, the reasons why word knowledge correlates with comprehension cannot be determined satisfactorily without improved methods of estimating the size of people's vocabularies. It suggests that improved assessment methods depend upon thoughtful answers to questions concerning what a word is, what it means to know the meaning of a word, and the most efficient way of estimating vocabulary size from an individual's performance on a sample of words. (MKM)