Mycorrhizas are mutualistic interactions that the majority of land plants establish with a heterogeneous group of soil fungi; their distri- bution and diversity have supported the suc- cess of plants on the planet. Among all different types of mycorrhizas, arbuscular mycorrhiza (AM) is the most ancient and the most common in host plants of all major crops. The functional core of AM is a finely branched fungal structure called the arbuscule. Arbuscules are hosted inside living root cells, within a specialized cell compartment that is generated through a precise sequence of molecular and cellular events. Over the last 10 years, the application of novel technologies, such as genome sequencing, high-throughput transcriptomics, and live cell imaging, has generated substantial advances in our knowledge of such events. Here, we pres- ent a synopsis of the recent literature on the interactions between AM fungi and their hosts, with an evolutionary-developmental focus on the intimate contact that develops between plant cells and fungal hyphae, in terms of molecular signaling, nutrient exchange, and cell organization.