A great fraction of the world's energy requirements are presently met through the unfettered use of fossil-derived fuels. However, due to the anticipated demise of these energy sources and the environmental and socioeconomic concerns associated with their use, a recent paradigm shift is to displace conventional fuels with renewable energy sources. Among various alternatives, biomasses have garnered tremendous interests as potential feedstock for clean energy production. While numerous biorefinery schemes and conversion technologies exist for the transformation of biomass into usable energy forms, they are not cost-efficient and economically viable to compete with the existing petroleum-refinery technologies. In particular, the recalcitrant nature of several feedstock presents a major technological obstacle for their processing and transformation. Providentially, the synergistic integration of various biochemical and bioprocessing technologies is aiding in the establishment of future biomass energy programs. This article reviews the state of the art and future challenges in the recent development of biomass and associated transformation technologies for clean production of biofuels. (C) 2012 Elsevier Ltd. All rights reserved.