This book reviews how the release of the citrus genome facilitates the investigation of ancestral species, the study of their complex biological features, and the genetic basis of agronomic traits of paramount importance for their sustainable cultivation. The first chapters discuss citrus origin and distribution, and the economic importance and varietal composition of the cultivated species, providing an overview of citrus and related genera genetic resources. The book then describes the role of traditional breeding techniques (for scion and rootstocks) as well as the potential of genomic breeding and innovative protocols for biotechnological approaches. The second part provides essential information on the genus Citrus, the attributes of pure citrus species, genetic admixtures, hybrids and citrus relatives, and on the horticultural classification of cultivated species, varieties and rootstocks. The third part then focuses on the different molecular mechanisms, covering various aspects of citrus biology, including the role of beneficial compounds of citrus fruits. In addition, it examines the molecular responses of citrus to abiotic stresses and to field and post-harvest diseases. Providing insights gained in recent years, it is a valuable guide for those who are interested in gene discovery, comparative genomics, molecular breeding and new breeding techniques. It is particularly useful for scientists, breeders and students at universities and public sector institutes involved in research for the citrus industry.