The aim of QUILT is to optimise the exploitation of the existing European UV-visible monitoring systems by which O3 and the related free radicals NO2, BrO and OClO can be measured. These monitoring systems include ground-based, balloon and satellite observations. QUILT is providing an assessment of the chemical ozone loss over the last decade and through 2000-2003. This is achieved through analysis improvements, consolidation of existing datasets and near real time integrations with chemical transport models.