Plants rely on the symbiosis with arbuscular mycorrhizal (AM) fungi to efficiently obtain mineral nutrients, especially phosphate, from the soil. In addition of their role in plant mineral nutrition, AM fungi can offer a range of benefits to their plant hosts in return for photosynthates. Most knowledge on the molecular aspects of this key symbiosis have been obtained from the plant side. However, with the recent availability of AM fungal genome sequences and transcriptomes major progress has been made to get insight into the fungal components that control this interaction. In this review we summarize these recent molecular insights, especially from the AM fungus Rhizophagus irregularis, with a focus on plant-fungus communication, (reciprocal) nutrient exchange, and the genetic make-up of AM fungi.