The use of DNA markers to define the genetic makeup (genotype) and predict the performance of an animal is a powerful aid to animal breeding. One strategy is known as marker-assisted selection (MAS). MAS facilitates the exploitation of existing genetic diversity in breeding populations and can be used to improve a whole range of desirable traits. DNA markers are, by definition, polymorphic, and the methods used to define DNA markers include restriction fragment length polymorphisms (RFLPs), microsatellites, and single nucleotide polymorphisms (SNPs). Linkage analysis, association analysis and analysis of gene function can be used to determine which polymorphisms are useful markers for desirable traits. Future prospects include the use of high throughput DNA microarray (DNA chip) technology which could revolutionize animal breeding in the next millennium.