The dune forms, which we term valley-marginal dunes, are found at the top of valley flanks, and are narrow, elongate forms with arcuate and straight planform elements which mirror the course of extant and ancient channels. Following a discussion of the major schemes used to classify different dune types, we describe the morphology and situation of these valley-marginal dunes. We suggest that whilst the basic morphometry of these dunes is similar to many other dune types, the topographic setting in which they occur is relatively unusual. We conclude that valley-marginal dunes are a previously unrecognized landform which cannot be fitted into current classificatory schemes. It is suggested that one reason for this may be the lack of an explicit temporal dimension in most dune classifications. Keywords: valley-marginal sand dune, south-west Kalahari, dune classification.