Technical Abstract: Marker Assisted Selection (MAS) is the process of using the results of DNA tests, together with phenotypic and pedigree information to assist in the selection of individuals to become the parents in the next generation of a genetic improvement program. The additional information provided by the DNA test results should improve the accuracy of evaluating the genetic merit of the individuals in the population, and hence, should improve the rate of response to selection. This increased accuracy of evaluation is available early in the life of the animal (before selection decisions must be made). The advantages of MAS are greater for traits that are difficult to improve with traditional methods: those that are low in heritability, difficult or expensive to measure, or that can only be measured in one sex, postmortem, or late in the productive life of the animal. DNA test information does not eliminate the need to collect phenotypes, but it reduces the number of phenotypes that need to be collected. MAS has the potential to reduce the impact of antagonist genetic correlations by concentrating selection intensity on those genes that affect one set of traits without undesirable effects on other traits. DNA tests that could be considered for use in an MAS program can be grouped into the following general categories: functional tests, association tests, and linked markers. Functional tests are easiest to use, but most expensive to develop, while linked markers are the least expensive to develop, but most challenging to utilize in MAS. In the short term, association tests are likely to be the most widely used category. Pedigree, phenotypes, and DNA test information should be evaluated simultaneously in a combined analysis, resulting in a value referred to as a marker adjusted estimated breeding value, which is the best estimate of genetic merit, considering all available information. DNA tests are likely to change (improve) over time. There are several examples of application of MAS in commercial livestock populations. Commercial DNA tests for quantitative traits are on the market for several species. As more tests become available over the next few years it seems likely that MAS will become increasingly important in livestock breeding.