This work presents and discusses the results of the research on the gasification of biomass in an atmospheric circulating fluidized bed, with a mixture of steam and oxygen as fluidization / gasification medium. The main objectives of this research were to investigate and improve the gasification process in order to produce hydrogen-rich gas from relatively dry woody and agricultural biomass. The targeted gas application was the production of liquid biomass-based transportation fuels, e.g., Fischer-Tropsch diesel or DME. Gasification experiments have been carried out in a test rig with a maximum thermal throughput of 100 kW fuel input equivalent. The main investigated process variables were the steam-to-biomass ratio, the stoichiometric oxygen-to-fuel ratio, the kind of mineral used as the bed material, the kind of gasified biomass, and the use of an additive material to counteract the bed agglomeration. Also the operability of the gasifier under various process conditions was investigated. The tar concentration in the raw gas was reduced below 2 g/mn3, and the H2 fraction in the gas increased to almost 40% (by volume) in the dry gas after changing the bed material from quartz sand to magnesite. Using a clay mineral additive effectively counteracted agglomeration when fuels with high alkali metal content in the ashes were gasified in combination with silica-rich bed materials. Parallel to the experimental part, a mathematical model of the circulating fluidized bed gasifier was developed, based on an existing model for coal combustion units. The newly implemented aspects were the biomass pyrolysis model and the kinetics of the main gasification reactions, including tar cracking. The model has been validated with the data obtained during the experiments.