AM fungi occur in soil and in association with roots as communities of organisms that may simultaneously interact with the roots of one or several co-existing plant species. Species of AM fungi differ in their mode of colonisation and their capacity to form hyphae in soil and within the root (Abbott et al. 1992). Although hyphal characteristics may be distinctive for some fungi (Dodd et al. 2000), they are not usually present as discrete organisms and are difficult to distinguish from one another within and on the surface of roots. Although the fungi may have markedly different characteristics, they appear to function in a similar manner, but with different levels of efficiency depending on their abundance as well as their intrinsic characteristics. Furthermore, their symbiotic response depends on environmental conditions and the relative abundance of other AM fungi associated with the roots of the same plant. The purpose of this review is to discuss the functional diversity of AM fungi and its significance in the context of interactions at root surfaces and the potential consequences of this for plant growth and plant community structure.