The Argo network of autonomous profiling floats will provide the first global views of the time-varying temperature (T) and salinity (S) fields of the upper ocean. Argo will serve a broad community of scientific and operational users, with objectives falling into several categories. It will provide a quantitative description of the evolving physical state of the upper ocean and the patterns of ocean climate variability, including heat and freshwater storage and transport. The data will enhance the value of the Jason altimeter through measurement of the subsurface vertical structure of T and S, plus reference velocity, with sufficient coverage and resolution for interpreting altimetric sea surface height variability. Argo data will be used for initializing ocean and coupled forecast models, for data assimilation and for dynamical model testing. A primary focus of Argo is seasonal-to-decadal climate variability and predictability, but many applications for high-quality global ocean analyses are anticipated.