Evergreen azalea is one of the most important ornamental shrubsShrub and potPots plants in temperate zones worldwide. Hundreds of evergreen azalea cultivarsCultivars have been bred based on the genetic diversityGenetic diversity of endogenous speciesSpecies in the genus Rhododendron L., the subgenus Tsutsusi, section Tsutsusi since the seventeenth century, the Edo period (1603–1867) in Japan. The cultivarCultivars groups such as Edo-kirishima, Kurume, Satsuki, Ryūkyū and Hirado have been established by inter-specific hybridizationHybridization and selectionSelection, and they developed a wide varietyVarieties of flowerFlower colorColor and flowerFlower form traits. Many cultivarsCultivars have been introduced to Western countries and utilized as breedingBreeding materials for potPots and garden azalea. The traditional azalea cultivarsCultivars have accumulated valuable germplasmGermplasm and gene resources through the hundreds years of breeding historyBreeding history. For azalea breedingBreeding, it is important to understand more about the genetic resourcesGenetic resources and history of Japanese azalea cultivarsCultivars. Most of azalea cultivarsCultivars currently in use are traditional cultivarsCultivars and cultivarsCultivars that have developed by crossing or sport mutationMutation based on limited genetic resourcesGenetic resources. Although geneticGenetic modification and editing have not been developed in azalea, there is still significant potential for crossbreeding that adequately utilizes genetic resourcesGenetic resources. Our research group have been focused on the azalea genetic resourceGenetic resources which contains diverse gene resources. And we have been promoting research on conservationConservation of the genetic resourcesGenetic resources, characterization of the traits, elucidation of inheritance and responsible gene, and new breedingBreeding applications of the useful trait. Novel and attractive cultivarsCultivars can be developed by conventional breedingBreeding that utilizes genetic resourcesGenetic resources accumulated over 300 years based on Japanese horticultural cultureCulture.