The taxonomic revision of Maréchal et al. (1978) has brought within the confines of a single genus crops which had previously been distributed over three genera: Vigna itself, the genus Voandzeia (now merged with it) and Phaseolus (re-defined). The genus, in the revised sense, is distributed in the warmer parts of both the Old World and the New; all the cultigens are of Old World origin although the cowpea and the mungbean are now more widely distributed.

The members of the genus commonly cultivated are the cowpea itself (Vigna unguiculata), the groundbean or Bambara groundnut (Vigna subterranea) and the group of Asiatic forms (commonly known as grams) Vigna radiata, V. mungo, V. angularis, V. umbellata, V. aconitifolia, V. glabrescens and V. trilobata. The last two are of peripheral interest; V. glabrescens is a polyploid and V. trilobata can perhaps best be regarded as a semi-domesticate.

The extent to which these different species are exploited has been determined largely by economic factors. The cowpea is the subsistence legume par excellence; it is grown extensively in Africa and India in the Old World, and in Brazil in the New (Rachie, 1985). It is only in the USA that the cowpea is primarily a commercial crop, where it is grown to meet the demand for ‘black-eyed peas’ (dry mature seed). Green mature seed is also used both canned and frozen.