Most plants rely on arbuscular mycorrhizal (AM) fungi to obtain scarce mineral nutrients, such as phosphate and nitrate, from the soil. Therefore, they establish an endosymbiosis with AM fungi, where the fungi are accommodated inside root cortex cells as arbuscules. This forms a symbiotic interface where the fungi deliver phosphate and other minerals to the plant in return for sugars and lipids. Most molecular insights into this key symbiosis have been obtained from the plant side. However, since the recent availability of a genome sequence of the model AM fungus Rhizophagus irregularis, more and more insight into the fungal aspects of the symbiosis are being gained. As a first approach, we have focused on the fungal transcriptome in different host plant species and at different stages of the interaction through the use of laser microdissection. This showed that the fungus adjusts its transcriptome in a host- and stage-dependent manner. One of the striking aspects of AM fungi is their ability to intracellularly colonize the vast majority of all land plants, indicating that they must be highly effective in dealing with the plant immune system. To study this, we have especially focused on small secreted proteins that may act as effectors in analogy to effector proteins from pathogenic fungi. Our recent insights into the involvement of AM effectors will be presented.