A brief historical review is given of the taxonomic treatment of infraspecific classification of cultivated plants. The inadequacy of hierarchical classification for polymorphous cultivated plant species is outlined. The rapid change in the range of cultivars as a result of breeding to adapt to changing cultivation practices, and the creation of new cultivars and crops, complicates such classification. Attention is focused on the use of cultivar groups for infraspecific classification. The scope of this system is illustrated with an example in Brassica rapa L.