Mycorrhizal fungi belong to several taxa and develop mutualistic symbiotic associations with over 90% of all plant species, from liverworts to angiosperms. While descriptive approaches have dominated the initial studies of these fascinating symbioses, the advent of molecular biology, live cell imaging, and "omics" techniques have provided new and powerful tools to decipher the cellular and molecular mechanisms that rule mutualistic plant-fungus interactions. In this article we focus on the most common mycorrhizal association, arbuscular mycorrhiza (AM), which is formed by a group of soil fungi belonging to Glomeromycota. AM fungi are believed to have assisted the conquest of dry lands by early plants around 450 million years ago and are found today in most land ecosystems. AM fungi have several peculiar biological traits, including obligate biotrophy, intracellular development inside the plant tissues, coenocytic multinucleate hyphae, and spores, as well as unique genetics, such as the putative absence of a sexual cycle, and multiple ecological functions. All of these features make the study of AM fungi as intriguing as it is challenging, and their symbiotic association with most crop plants is currently raising a broad interest in agronomic contexts for the potential use of AM fungi in sustainable production under conditions of low chemical input.