With the rapid development of genome maps of livestock, many genetic markers are becoming available. These markers usually have no affect on animals in their own right, their value is that they allow the inheritance of sections of chromosome to be followed in suitable pedigrees. How then may genetic markers be used in livestock breeding? It is useful to distinguish the use of markers in selection programmes within breeds and crossbreeding programmes. Furthermore, for both types of breeding programmes there are different kinds of information that can be provided by markers. Genetic markers may be linked to major genes, or in some cases we may have a direct test for a mutation which causes a major difference in animal performance or phenotype. Alternatively, we may have markers which are linked to (unknown) quantitative trait loci (QTLs), i.e. loci which influence traits such as growth, milk yield, carcass quality and litter size.