Mastitis, an inflammatory response associated with bacterial infections, is generally regarded as the most-costly disease of dairy cattle because of its high incidence and effects on milk production and composition. Genetic selection for highly productive dairy cows has been very successful; however, udder health has declined in many dairy breeds because of its unfavourable correlations with milk production. Poor udder health increases veterinary and farm labour costs, increases rates of involuntary culling, decreases farm revenue, and adversely impacts animal welfare. However, genetic selection can be used to improve udder health just as it has been used to increase production. This chapter reviews advances in dairy cattle breeding to improve resistance to mastitis. It includes sections on both conventional and new phenotypes for improving resistance to clinical mastitis, and also on both national and international genetic improvement programmes for resistance to clinical mastitis. It concludes with a section on increasing rates of genetic gain through genomic selection.