"Argo is an international array of 3000 profiling floats that measure temperature and salinity throughout the deep global oceans, down to 2,000 metres. It is the single most important global in-situ observing system for the GMES Marine Service. Argo provides critical observations of the ocean interior that are required to constrain, together with satellite observations, GMES Marine Service modelling and forecasting systems. The European long-term contribution to Argo is organized as part of the Euro-Argo research infrastructure that will become in early 2012 a new European legal entity. The main challenges for Argo and Euro-Argo are 1/ to maintain the global array and ensure its long term sustainability and 2/ prepare the next phase of Argo with an extension towards biogeochemistry, the polar oceans, the marginal seas and the deep ocean. Meeting such challenges is essential for the long term sustainability and evolution of the GMES Marine Service. This requires major improvements in Argo float technology. New floats with improved capabilities are or will be soon available from float manufacturers. They require, however, extensive testing at sea before they can be used for operational monitoring. The Euro-Argo data centers need also to be upgraded so that they can handle these new floats. E-AIMS will organize an end-to-end evaluation of these floats (from float design down to the use by GMES). Observing System Evaluations and Sensitivity Experiments will also be conducted to provide robust recommendations for the next phase of Argo that take into account GMES Marine Service, seasonal/decadal climate forecasting and satellite validation requirements. E-AIMS will thus demonstrate the capability of the Euro-Argo infrastructure to conduct R&D driven by GMES needs and demonstrate that procurement, deployment and processing of floats for GMES can be organized at European level. These are key aspects for the long term sustainability of GMES in-situ component."