The bulb or common onionOnion is the most cultivated species of the genus Allium worldwide. The basal portion of the leaves has swollen forming the bulbs when a certain day-length is reached. The bulb composed of reduced, compacted, underground stems surrounded by modified fleshy scale (leaves). The scales encircle a central bud located at the tip of a reduced stem. Due to causing serious crop damage, theOnion breeding chief onion breedingBreeding objective is the pests and diseasesDiseases resistance/tolerance. Onion improvement to cope with climatic changes is one of the core current breeding goals. Onions are cultivated and utilized around the year. Therefore, bulb adaptability and storability are important. In spite of existing useful inter-and intra-specific useful traits, their introgression is very limited due to genomic incompatibility resulting in partial or complete sterility. Generally, there is reluctance concerning onion breedingBreeding as it is open pollinated biennial crop and suffers inbreeding depression only after 2–3 cycles of selfing, the maximum. Male-sterility is available in onion, however, breeding highly inbred parental lines for F1 hybrid (heterozygous/homogeneous breedingBreeding) cultivar development is difficult if not possible at all. Consequently, population breeding (heterozygous/heterogeneousBreeding breeding) is still dominating in onion improvement. Double haploids can play an effective role to overcome the limitation of breeding pure lines. F1 hybrids industry is advantageous in terms of protecting the breeder rights but it can cause genetic erosionGenetic erosion. Therefore, newly emerged omics technology may be beneficial for identification, characterization and conservationConservation of the germplasmGermplasm. Development of saturated genetic map andMarker-assisted selection marker-assisted selectionSelection holds a promise to use in onion genetic improvementGenetic improvement. OnionOnion has a relatively, huge genome sizeGenome size and this is a disadvantage, especially for its functional genomic analyses and for its genetic transformationGenetic transformation.