The objective of the Hydros Earth System Science Pathfinder mission is to collect global scale measurements of the Earth's soil moisture and land surface freeze/thaw conditions, using a combined L band radiometer and radar system operating at 1.41 and 1.26 GHz, respectively. Brightness temperature data from the Hydros radiometer will have a nominal resolution of ∼40 km and an incidence angle of ∼40 degrees. Along with the inclusion of needed static and dynamic ancillary data, the Hydros calibrated TB will be used to generate an Earth-gridded derived soil moisture product. Members of the Hydros team are currently in the process of evaluating possible candidate soil moisture retrieval algorithms in order to select a baseline algorithm by mid-2006, although the algorithm test and evaluation process will likely continue through the Hydros launch in 2010. The accuracy of the various ancillary data parameters, as well as their temporal and spatial resolution, time lag, and ease of availability, will be an important factor in driving the overall error budget for the Hydros soil moisture products.