Modeling of biomass gasification in bubbling and circulating fluidized bed (FB) gasifiers is reviewed. Approaches applied for reactor modeling, from black-box models to computational fluid-dynamic models, are described. Special attention is paid to comprehensive fluidization models, where semi-empirical correlations are used to simplify the fluid-dynamics. The conversion of single fuel particles, char, and gas is examined in detail. The most relevant phenomena to be considered in modeling of FB biomass gasifiers are outlined, and the need for further investigation is identified. An updated survey of published mathematical reactor models for biomass and waste gasification in FB is presented. The overall conclusion is that most of the FB biomass gasification models fit reasonably well experiments selected for validation, despite the various formulations and input data. However, there are few measurements available for comparison with detailed model results. Also, validation of models with data from full-scale FB biomass gasification units remains to be done. (C) 2010 Elsevier Ltd. All rights reserved.