The overall objective of COSE is to provide the Earth Observation (EO) user community with a validated, consistent and well-documented data set of mainly stratospheric constituent columns and/or profiles, by co-ordination of ground-based observations at existing stations in Europe. The data set builds on past and ongoing time series, and will be archived in a dedicated database for immediate and future exploitation, e.g., satellite validation activities, data assimilation and scientific studies. Active participation of some representative EO customers will assure that the delivered data sets come up to their requirements.