Biology, diversity and tools for diversity analysis of the onion (Allium cepa L.). A review. Description of the subject. This study explores important information on biology, genetic resources, taxonomy, as well as morphological, biochemical and molecular markers in order to provide a better understanding of the genetic diversity of the onion (Allium cepa L.). The study has a particular focus on the plant as grown in Africa. The onion is a monocotyledonous, allogamous, and entomophilous plant, with a one-year production cycle for bulb production, and a two-year production cycle for seeds. The onion is one of the most significant vegetables in the world because of its use as both a food and a medicine. Literature. Twenty-eight morphological markers, linked to seed, leaf, flower and bulb traits, were identified as the most discriminant phenotypical criteria. Biochemical and molecular markers were also developed to characterize genetic variations between and within onion varieties. Previous studies examining West Africa onion varieties showed that only six enzyme systems are polymorphic. However, only twenty-four isozymes have been used to compare the onion to other Allium species. This low number of polymorphic biochemical markers makes it more difficult to determine the genetic diversity of onions. On the other hand, molecular markers at DNA level, such as RAPD, RFLP, AFLP, SSR, are very useful to analyze diversity at varietal and species level, using cultivated and spontaneous forms, and to analyze the level of introgression between the onion and the other species of the genus. Conclusions. Genetic diversity analysis showed an important variability between and within Africa onion landraces. It would be useful to combine in situ and ex situ conservation, using these genetic resources to improve the production and the appropriate use of African onion cultivars.