Galium aparine L., cleavers, is considered to be native, as well as introduced into North America from Eurasia. It is found from southern Alaska, across the wheat belt of Canada, to Newfoundland. Galium spurium L., false cleavers, is solely an introduced weed. Both species have several characteristics that predispose them toward weediness. These include rapid seedling development, early flower initiation after a short period of vegetative growth, self-compatibility, "stickiness" of fruits and plant foliage which aids in dispersal and crop contamination, resistance to phenoxy acetic herbicides and seedling emergence throughout the growing season, which helps the plants escape herbicides and cultivation. Both species are found in grain fields but examination of contaminated seed so far has shown only G. spurium to occur in rapeseed. Galium spurium is the more weedy and aggressive of the two species and is better adapted to growing conditions on the prairies.Key words: Weed biology, cleavers, false cleavers, Galium aparine, Galium spurium