This chapter introduces arbuscular mycorrhizal fungi (AMF), addressing four key aspects: their biological characteristics, root colonization processes, and molecular features. AMF are obligate symbionts that form mutualistic relationships with numerous plants, enhancing growth, resistance, and tolerance to abiotic and biotic stresses. They cannot grow independently in nature or axenic culture without host plants. Their main structures include arbuscules, vesicles, auxiliary cells, hyphae, and spores. Root colonization involves genetic, morphological, and functional interactions between the plant and AMF, starting even before physical contact. At the molecular level, AMF exhibit significant genomic variation across species. For instance, Rhizophagus intraradices has a small genome (~16.54 Mb, with 88.36% single-copy DNA, 1.59% repetitive sequences, and 10.05% foldback DNA), while species like Scutellospora pellucida and Racocetra gregaria have much larger genomes, ranging from 127.4 to 1058.4 Mb, respectively.