Ethanol production from lignocellulosic hydrolysates in an economically feasible process requires microorganisms that produce ethanol with a high yield from all sugars present (hexoses as well as pentoses) and have a high ethanol productivity in lignocellulosic hydrolysates, i.e., can withstand potential inhibitors. Different fermentation organisms among bacteria, yeasts, and fungi (natural as well as recombinant) are reviewed with emphasis on their performance in lignocellulosic hydrolysates. Depending oil the type of lignocellulosic hydrolysate, the composition of inhibitors will differ and their influence on the microorganisms and the fermentation performance will consequently vary. The inhibition may be partly overcome by the removal of inhibitors, i.e., detoxification. Microbial constraints on parameters such as pH, temperature, and nutrient supplementation ave discussed in relation to their implication on the process economy. Nor only are the properties of the microorganism of importance in the process, but also the choice of fermentation strategies such as batch culture, continuous culture with cell recycling and in situ ethanol removal. For the realization of the ethanol production from lignocellulosic materials, the fermentation step has to be integrated with the rest of the process. These aspects are also discussed.