Second-generation feedstocks are a potential source of fermentable sugars; they offer great advantages in reducing the cost of production, greenhouse gas emissions, and competition with food and agricultural lands. Fermentation processes for production of different biofuels from these feedstocks differ but the prefermentation processes are similar. Effective fermentation processes are a prerequisite for biofuel production in large capacity as well as making its production economically competitive with the price of crude oil. This chapter addresses various fermentation processes for biofuels; major pathways, fermenting microorganisms, challenges associated with the specific fermentation processes, and possible solutions. In addition, process optimization strategies and bioreactor design suitability for effective fermentation processes are described. © 2019 Angelo Basile and Francesco Dalena Published by Elsevier Inc. All rights reserved.