To better understand and model the observed variability of the earth system, one can rapidly expand the available record by objectively combining disparate observations with numerical model-generated guesses. The "reanalyses" or "syntheses" of atmospheric, oceanic, land, and cryospheric observations generated by such a data assimilation process provide comprehensive physically consistent descriptions of evolving system states. Such widely used datasets have become essential to the study of the earth system on timescales from hours to a century. However, they are not without room for improvement. This session welcomes all contributions on applications of reanalysis data, on comparisons of reanalyses and observations, on advancing reanalysis methodology, and on issues in using reanalysis for weather and climate problems.