Genetic selection has proved itself to be the most powerful tool available for the improvement of livestock. Over the last 40 years the efficiency of livestock production has been increased radically by the success of animal breeding. Where animal breeders have been successful it has been through the application of relatively simple rules – selection based on traits that can be measured in a repeatable manner and where variation between individuals has both a genetic component and economic relevance. Until recently, this has meant that breeders have focussed on efficiency traits – for example growth rate or milk yield - and in some cases, certain easily measured aspects of quality, such as overall fatness as inferred from ultrasonically measured fat depth. As both the breeding industry and consumers have become more sophisticated, there has been an increasing focus on traits associated with reproduction, welfare and disease and quality. Such traits can be more difficult to improve by selection because heritabilities are low (i.e. only a small amount of variation between animals is genetic in origin, such as for some reproduction and disease associated traits). In addition, such traits may be difficult or expensive to measure (e.g. disease related traits, or quality related traits measured in the laboratory or by sensory panels) and have economic values that are difficult to quantify.