Persistency of perennial ryegrass is lower as compared with that of other cool temperate grasses because it is more susceptible to summer and winter stresses. To raise its persistency, it is required to increase not only resistance to summer damages caused by drought, high temperature and diseases, but also to winter losses caused by cold temperature and snow mold disease. Some cultivars are practically free from crown rust and have a high-yielding capabilities in the warm regions. Their present level of resistance to other stresses, however, are still to be improved yet to raise their persistency. Resistance to snow mold is independent from the resistance to summer stresses, while it is correlated with cold hardiness. The method employed for testing cold hardiness in the breeding program of the Yamanashi Prefectural Dairy Experiment Station, Japan, proved to be also effective for improving snow mold resistance. However, none of the selections for resistance to summer stresses showed higher levels of resistance to drought and high temperature than the existing cultivars. However, genetic variations within the strains or cultivars were much larger than those among them. Taking those variations into account, a maternal line selection method was adopted to improve stress resistance. Since there was a significant correlation between the parents and their offsprings, the maternal line selection would be an effective procedure. For the purpose of improving the adaptability of perennial ryegrass to warm regions in Japan, a great number of introductions from diverse origins are presently under test, including an attempt to introduce new genes from tall fescue. Some materials would be promising in achieving greater adaptability.