DNA markers are rapidly being developed for almost all the major crops. The most important markers are restriction fragment length polymorphisms (RFLPs), polymerase chain reaction (PCR) based markers such as random amplified polymorphic DNA (RAPD), and fingerprinting markers. DNA markers can supplement isozyme markers for monitoring trait improvement activities such as estimating genetic diversity in breeding populations, germplasm identification, verifying controlled crosses, and estimating seed efficiencies. As the number of DNA markers is potentially limitless, it should be possible to map individual quantitative trait loci (QTL) by linkage analysis with high-density maps. Twenty-first century agriculture will face frightening challenges to provide mankind with an appropriate level of food security while enhancing the sustainability of agricultural practices, lowering their environmental impact and preserving the remaining biodiversity. Marker assisted selection (MAS) have been widely adopted to improve resistance to biotic stresses, more modest results have been reported for the improvement of resistance to biotic constraints particularly drought and yield, mainly due to the elusive nature of the applicable quantitative trait loci (QTLs) and the unpredictability of their effects. In this article, what Marker assisted selection (MAS) is and why it is a good idea is described. MAS will probably exhaust genetic variation more hurriedly than phenotypic selection because many more cycles of selection are possible in a given time period using genomic compared to phenotypic selection.