Biomass Chemical Looping Gasification (BCLG) has emerged in last years as a promising technology to produce high-purity, renewable syngas with many end-uses including transformation into chemicals and biofuels. This work summarizes the expertise of the ICB-CSIC on the development of the technology including oxygen carrier screening and tests in continuous units at scales of 1.5-20 kWth. Synthetic oxygen carriers have shown improved characteristics in terms of performance and syngas quality, with respect to the use of minerals. Especially relevant were the results obtained with a Cu-based material, versus the based-on Fe or Mn, which demonstrated to have a high lifetime (+5000 h) and catalytic properties that leads to high CH4 conversion and reduce tar contents. Several experimental BCLG campaigns were carried out at about 20 kWth scale unit inside the EU H2020 CLARA and BioCLG projects. Different biomasses were used as fuel feedstock (wheat straw, pine forest residue and torrefied wheat straw) and ilmenite as solid oxygen carrier. The operational variables analyzed were the fuel reactor temperature, the mean residence time of solids in the fuel reactor, and the oxygen-to-biomass ratio. Control of the oxygen fed to the air reactor proved to be a simple and robust method to control the oxygen used for syngas production. More than 450 hours of BCLG operation were reported in continuous units, of which ~100 h corresponds to 20 kWth. A comparison of the results obtained at the different scale was finally done to infer the main operating parameters affecting process optimization.