For most diseases of economic importance in livestock, resistance can be considered to be a complex or quantitative trait with limited evidence of negative correlations with resistance to other diseases. Attention to such traits allows the selection of healthier and more productive animals. This chapter reviews the sources of variation in resistance to disease, and then considers three strategies for selecting for resistance: by directly selecting for resistance to important diseases, based on the incidence of disease or a disease-related marker; by selecting for animals with strong innate and/or adaptive immune responses to a specified challenge to achieve a broad-based disease resistance; and by selecting for animals that perform well in an environment in which disease is endemic. The discussion also covers high-density SNP chips, improved sequencing methodologies and genome-wide association studies (GWAS)