The most widespread and prominent symbiosis between land plants and fungi is the arbuscular mycorrhiza (AM). This type of mycorrhiza symbiosis is formed between approximately 80% of land plants and a monophyletic group of obligate symbiotic, multikaryotic and asexual fungi, the Glomeromycota. Despite the enormous ecological and economical importance of AM fungi, their biology is poorly understood. The focus here is, after reporting some historical aspects, on the recently advanced understanding of molecular phylogenetic relationships, the evolution and biogeography, and the obligate symbiotic endobacteria of AM fungi. Fossils and molecular clock estimates date the origin of AM symbioses to at least 460 MY ago, and AM fungi and land plants coevolved since then. Possibly, the fungi in the Glomeromycota were already ‘on the symbiotic track’ long before land plants originated. Aspects regarding the asexual evolution and heterokaryotic nature of glomeromycotan fungi and a feasible species concept are discussed.