Arbuscular mycorrhizas (AM), mutualistic plant–fungus symbioses formed on the majority of land plants, appear to play an important role in plant mineral nutrition, plant health, and might influence the structure of plant communities. Progress in understanding the genetic and molecular basis of this important symbiotic association has been hampered by the obligate biotrophy of the fungal partner, and by the lack of mycorrhiza formation on the plant model species Arabidopsis thaliana. Recently, legume plants, such as Medicago truncatula and Lotus japonicus, have been chosen as experimental systems for research of plant–microbe symbioses. The application of both reverse- and traditional forward-genetic approaches, in conjunction with genomics initiatives in model legumes, will rapidly increase our understanding of the plant-encoded genetic determinants of the arbuscular mycorrhiza. An essential step in this process has been the generation, isolation and analysis of mycorrhizal mutants. This review provides an introduction to existing plant mutants affected in AM formation.