The ubiquitous nature of arbuscular mycorrhiza (AM) pleads for common molecular and genetic determinants across different plant taxa. The cellular processes determining compatibility in early interactions prior to and during cell contact between arbuscular mycorrhizal fungi and plant roots are starting to be unravelled. The root epidermis is an active checkpoint where signal exchanges and control over root colonization occur. Root-secreted flavonoids, flavonols, and strigolactones can act as rhizosphere signals in stimulating presymbiotic fungal growth, although their mechanism of action on the fungal cell is as yet unknown. Likewise, fungal signals (Myc factors) activate early plant responses with induction of genes related to signal transduction pathway and biogenesis of a prepenetration apparatus designed to accommodate intracellular fungal growth from appressoria into epidermal cells. Evidence from genetical, transcriptional, and physiological studies points to the implication of calcium as a secondary messenger in signalling pathways leading to early host cell responses. Future challenges for research are to decipher the complexity of symbiosis signalling and to provide new insights into the specificity of the molecular dialogue between AM symbionts.