Symbiosis is common among many plants, animals and microorganisms. Symbioses between plants and fungi are particularly widespread and the relationship between the symbionts can span the range between mutualism and parasitism. These symbioses have the potential to influence major ecological processes and also be an important determinant of plant diversity in ecosystems, either by altering the species composition of plant communities or by changing the genetic structure of plant populations. In this manner, symbiotic fungi could influence the way that their plant hosts evolve. Global environmental changes such as elevated atmospheric carbon dioxide or habitat fragmentation could alter the balance between symbiotic fungi and their partners and consequently alter the diversity or structure of natural plant communities and populations. A deeper insight into the functional diversity of fungal symbionts is also important if they are to be considered in future sustainable agriculture practices.