Once launched in late 2014, NASA’s Soil Moisture Active Passive (SMAP) mission will use a combination of a four-channel L-band radiometer and a three channel L-band radar to provide high resolution global mapping of soil moisture and landscape freeze/thaw state every 2-3 days. These measurements are valuable to improved understanding of the Earth’s water, energy, and carbon cycles, and to many applications of societal benefit. In order for soil moisture and freeze/thaw to be retrieved accurately from SMAP microwave data, prelaunch activities are concentrating on developing improved geophysical retrieval algorithms for each of the SMAP baseline products using data from simulations, from existing satellite missions such as SMOS, and from field campaign data, such as the SMAPEx airborne study in Australia discussed in this paper. ; J.P. Walker, P. O'Neill, X. Wu, Y. Gao, A. Monerris, R. Panciera, T. Jackson, D. Gray and D. Ryu