Copernicus Land Monitoring Service (CLMS) produces biogeophysical maps of the global land surface. The CLMS portfolio so far did not include actual evapotranspiration (ETa), despite it being a direct link between the energy, water and carbon cycles and its importance for global food security, efficient water resources management and weather forecasting. However, a global CLMS ETa product is currently under development and will enter operational production by the end of 2025. It will have a spatial resolution of 300 m, dekadal (10-daily) temporal resolution, will consist of evaporation and transpiration sub-products and (like all other CLMS products) will be distributed under free and open data policy. It will be based mainly on Copernicus input data with primary satellite imagery coming from the observations of OLCI and SLSTR sensors on board of Sentinel-3 satellites. Such product will fill a gap in currently existing global and operational ETa products, thus satisfying a wide range on potential users' needs. In this paper, we describe the various design choices taken during the development of the ETa product, ranging from cloud masking and gap-filling, through derivation of biophysical traits, radiation components and weather forcings to spatial sharpening of the land surface temperature observations. Those data were then used to drive two evapotranspiration models: TSEB-PT and ETLook. A prototype implementation of the ETa processing chain was used to produce ETa data across a globally representative range of climatic zones and plant functional types, which was validated against measurements from 104 Eddy Covariance flux tower sites. The resulting overall best root mean squared error (RMSE) of 0.80 mm/day (relative RMSE of 47 %), bias of -0.12 mm/day (relative bias of 7 %) and coefficient of determination of 0.84 compare well with a similar global ETa dataset and are encouraging for the upcoming operational production of ETa maps.