Most research on the acquisition of second language vocabulary has depended on estimates of vocabulary size, or 'breadth' measures, rather than on estimates of 'depth' defined either in terms of kinds of knowledge of specific words or in terms of degrees of such knowledge. Breadth tests provide rough comparative estimates of individuals' overall vocabulary knowledge, useful for such purposes as placement in instructional programs, and for charting group gains for purposes such as program evaluation. However, such measures have a number of limitations, an obvious one being that they do not measure how well given words are known (Read, 1988); thus they are of limited value in studies of the vocabulary acquisition process or in assessment of curriculum-related vocabulary learning. Few procedures and even fewer test instruments have been proposed that attempt to carry out these functions. This article surveys existing second language (L2) vocabulary measures of both kinds and describes in detail a recently developed instrument designed to assess levels of familiarity with given words, the Vocabulary Knowledge Scale (VKS) (Paribakht & Wesche, 1993a, 1993b).