The onion (Allium cepa L.) is a vegetable crop and is the most widely cultivated species of the genus Allium. This genus also contains several other species cultivated for use a human food. This book reviews the applications of biochemical and genetic markers for assessing the genetic variation and phylogenetic relationships of onion germplasm in order to provide developed technologies for crop improvement. This book is of interest to plant geneticists, plant breeders, researchers, and scientists.