Discovery of an efficient bioconversion of cellulosic biomass and its hydrolysis to ethanol is the key to unlocking in developing of a bioethanol industry. The lack of industrially suitable microorganisms to convert xylose to ethanol fuel has been cited as a major technical bottleneck. In the past decades, many improvements have been made in the metabolic engineering of microorganisms, including Zymomonas mobilis, Escherichia coli, and yeasts, for the fermentation of xylose to produce ethanol by introducing genes for either xylose metabolism or ethanol production. The history and the current progress in constructing these strains are presented in this review.