VAM are important in the growth of cotton and the production of lint. VAM are a normal part of cotton plants and they are usually present in the roots. Only in their absence do we realise their importance. In syndromes like Long Fallow Decline or growth decline following land-planing, reduced plant growth and cotton production coincide with reduced VAM in roots, and reduced phosphorus content in mature plants. While we realise their importance, VAM are difficult to study. VAM are an association between soil-borne fungi and the roots of most plants. For the fungi, the association is obligate, and growth of the fungi relies entirely on the host plant. We cannot see the fungi in soil, nor in the roots of the plant unless we use laboratory based processes. Thus study of the fungi and their effect on plants requires specialised equipment and trained staff. CRDC has funded VAM research for several years, This paper summarises the research on the population biology of VAM fungi in cotton soils in eastern Australia completed at the University of Sydney.