A review of the literature on a variety of issues related to testing vocabulary knowledge in a second language addresses these topics: problems in estimating vocabulary size, including the related questions of what constitutes a word, how a sample should be selected, and what are the criteria for knowing a word; sampling the basic and specialized vocabulary of second language learners; diagnostic use of tests of vocabulary knowledge and the construction of such tests; the checklist as an alternative diagnostic measure; problems related to testing words in isolation; and the distinction between receptive and productive vocabulary knowledge. (MSE)