Marker-assisted selection (MAS) is widely used by soybean [Glycine max (L.) Merr.] breeders in both the private and public sector in the U.S. An example of this widespread use is a report by Pioneer Hi-Bred stating that they collected two million marker data points in 2003 as part of their soybean breeding effort. The wide use of markers by the soybean breeding industry is at least partially the result of public investment in developing marker systems and in identifying associations between markers and genes. The public soybean map consists of over 1,800 genetic markers and these are mostly RFLP and SSR markers. Efforts are currently underway in the public sector to develop and map SNP markers. Both private and public soybean breeders have had their greatest success in finding marker-trait associations and in conducting MAS for disease resistance. For example, MAS has been especially useful in breeding for resistance to soybean cyst nematode (Heterodera glycines Ichinohe) because this resistance has relatively simple inheritance and is expensive to phenotype. A current focus of the soybean breeding program at the University of Illinois is the development of cultivars with soybean aphid (Aphis glycines Matsumura) resistance. In 2004, we mapped a major aphid resistance gene and by the successful application of MAS during backcrossing, we developed aphid resistant lines through three backcrosses. These backcross lines were yield tested in the field in 2006. This rapid progress was due to the quick cycling of generations in a greenhouse and the use of MAS during each backcross generation and among plants in segregating populations.