Biomass gasification is a promising technology in current and future low carbon energy systems. Soot formation is a great technical challenge for the industrialization of biomass gasification that is inevitable at high temperature and fuel rich conditions. In this review, a comprehensive summary of soot formation in biomass gasification is provided with special focus on entrained flow technologies. The topics covered the state of the art knowledge of soot formation in different gasifiers, the fundamental knowledge, experimental methods and recent control strategies. Soot generation and oxidation mechanism are discussed while the relationship between soot, tar and char in biomass gasification are analyzed in detail. Reaction models for soot formation coupled to the gasification process are introduced, including (semi-)empirical and detailed models. Effect of biomass components and ash forming elements on soot formation are highlighted. This is followed by a detailed description of in-situ and ex-situ experimental measurements, such as the optical diagnostics, aerosol particle mass analyzer and mass spectrometer. Soot formation characteristics and properties in different types of gasifiers are then addressed in detail with an emphasis of entrained flow gasifiers. Finally, the soot control strategies in biomass gasification are reviewed and evaluated. This review concludes by summarizing the available knowledge and challenges in soot formation during biomass gasification.