Mycorrhizas are associations between plant roots and specialised soilborne fungi. Plants provide photosynthates to the fungi, which increase the access of plants to poorly available nutrients. Arbuscular mycorrhizas are the most common of these plant-fungus symbioses. Traditionally, the association was considered mutualistic when single plants showed an increase in growth, and parasitic when no effect or growth depressions were observed. Thus, where fungi did not provide any benefit to the plant they were considered ‘cheaters’. These ideas could not explain the persistence of the symbiosis in plants (crops or non-cultivated) that do not have an apparent benefit from arbuscular mycorrhizal (AM) fungi, or the outcome of interactions between positively responsive and non-responsive (including non-host) plants. Recent findings indicate that: (i) the contribution of AM fungi to plant nutrition may be large, but remains hidden unless special techniques are used; (ii) plant responses depend strongly on the plant-fungus combinations (diversity in function of AM symbiosis); and (iii) plant density, AM status of the competing plants and plant-fungus combinations affect the outcome of interactions between AM and non-mycorrhizal plants. We present an overview of AM symbiosis, discuss these new findings and present a brief insight on the role of the symbiosis on plant-pathogen interactions.