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8Nowadays concerns regarding fossil fuel resources depletion as well as environmental issues 9 attributed to CO 2 accumulation in the atmosphere force communities toward utilizing 10 biomass as a substitute fuel source which is environmentally secure and renewable. Pyrolysis 11 bio-oil from biomass comprised varieties of undesirable oxygenate and heavy compounds 12 have to be treated before its feeding to bio-refineries. Catalytic biomass pyrolysis vapor 13 upgrading presently seems to be a techno-economical process toward production of fuel-like 14 components. However, selection of stable and productive catalyst (s) to yield desirable 15 chemicals with low coke formation is a great challenge. Three most important classes of 16 catalysts comprising zeolites, mesoporous catalysts and metal based catalysts are utilized for 17 vapor phase bio-oil upgrading. This study dedicates a comprehensive review on catalytic 18 biomass pyrolysis vapor upgrading by emphasizing particularly on catalyst's types and 19 properties, coke formation over catalysts and catalytic process conditions.20