In the second edition of this book, we wanted to retain the unique angle of our original goal of bringing the reader a state-of-the-art multidisciplinary view of arbuscular mycorrhiza (AM) – plant interactions, beginning with pre-penetration stages, through colonization, symbiotic function, influence of the symbiosis on the host and its surroundings, and ending with evolution and diversity of mycorrhization. To this aim, we have recruited leading scientists in their fields to provide a thorough and detailed review of the current status of scientific knowledge on mycorrhizal biology. The subject of host–fungal interactions during the pre-penetration stage of AM is discussed in the first four chapters of the book. The first chapter, a review by Manuela Giovannetti, Luciano Avio and Cristiana Sbrana, presents the physiological and genetic aspects of AM fungal spore germination and pre-symbiotic mycelial growth. This chapter focuses on new insights into the main fungal developmental switches in the absence of the host; it promotes resolution of an interesting enigma: despite a putative strong selective disadvantage, how do these obligate symbionts compensate for the lack of host-regulated spore germination?