Many by-products are generated during gasification, such as tar, NOx, SO2, and fly ash. In particular, tar elimination from the product gas is necessary to make gasification an attractive option. The presence of tar can cause operational problems to further equipment; heavy tars may condense on cooler surfaces downstream which can lead to blockage of particle filters and fuel lines. With the aim of establishing a mechanism for tar formation, tar precursors were identified based on biomass main components - lignin, cellulose and hemicellulose. This review describes the fundamentals of the possible mechanisms for tar formation and evolution, as well as the background for the development of a model for the simulation of a biomass fluidised bed gasifier. (C) 2013 Elsevier Ltd. All rights reserved.