Fast pyrolysis of biomass is one of the most promising technologies for converting biomass to liquid fuels. As a result, this technology has gained extensive attention in the last two decades. The pyrolysis liquids, known as the bio-oils, have been regarded as promising candidates to replace petroleum fuels to be used in various thermal devices. However, bio-oils are totally different from petroleum fuels. Therefore, the successful utilization of bio-oils in terms of liquid fuels requires adequate understanding of their fuel properties. This review concentrates on the detailed fuel properties of bio-oils and discusses how these properties affect the utilization of bio-oils. (c) 2009 Elsevier Ltd. All rights reserved.