Lignocellulosic raw material primarily consists of lignin, cellulose, and hemicellulose. Lignin is a heterogenous polymer of substituted aromatic building blocks, cellulose is a linear polymer of glucose and hemicellulose is branched polymer, which in addition to glucose, may also contain the hexose sugars mannose and galactose, and the pentose sugars xylose and arabinose. Pentose sugars are primarily present in hemicellulose derived from hard woods, agricultural residues and grasses. In such raw materials pentose sugars can make up more than 35% of the total dry matter. Technical and economical assessments have estimated that the complete conversion of pentose sugars to ethanol would reduce the production cost of bio-ethanol by as much as 22%. Therefore a considerable research effort has been made over the past 20-30 years to construct and develop microorganisms that efficiently ferment both hexose and pentose sugars to ethanol.