Gasification of biomass is a promising source of fuels and other chemical products. However, the removal of tars, ammonia, hydrogen sulfide, and other byproducts from the raw gas is required. The gas clean-up technology that offers more advantages is hot catalytic gas conditioning downstream of the gasifier reactor. Here, we review the applications of basic, acidic, metallic, and redox catalysts for the removal of such byproducts and research strategies to prevent coke accumulation on the catalytic surface, to increase the resistance to sulfur poisoning, and to bring the by-product removal reactions to temperatures below 600 degrees C.