EC-Earth is a new Earth System Model (ESM) based on the operational seasonal forecast system of the European Centre for Medium-Range Weather Forecasts (ECMWF). Climate and weather forecasting applications share a common ancestry and are build on the same physical principles. The emerging concept of ‘‘seamless prediction’’ forges weather forecasting and climate change studies into a single framework. Employing this concept, EC-Earth benefits greatly from the advances in Numerical Weather Prediction (NWP) modeling. Even though ESMs like EC-Earth are used for different purposes and time scales compared to NWP models, the quality of long-term predictions relies heavily on accurate simulations of slow components of the Earth system, on precise incorporation of the many interactions between fast and slow processes in the Earth system, and on initialization procedures.