Native and non-native pathogens, insects and animals continue to have serious negative impacts on forest ecosystems and planted forests worldwide. Climate change will alter host-pest relationships and may increase detrimental impacts from many biotic agents. Genetic variation in resistance within tree species is a key element to maintaining forest health. It is the primary natural method of tree defense and utilizing genetic resistance is one of the few viable management options available to combat the adverse impacts of pests. Resistance research programs, including resistance breeding, are vital since they increase the efficiency in utilizing genetic variation to restore forest health when mortality becomes unacceptably high.