Assimilation of satellite-derived rainfall and moisture products from passive microwave measurements has been shown to significantly improve short-range weather forecasts and the quality of global analyses of moisture and vertical motion. Directly assimilating radiances, however, offers additional benefits such as not being subject to biases from initial guesses and a priori assumptions and allowing for control of background errors in the assimilation environment. This research (supported by the Joint Center for Satellite Data Assimilation (JCSDA)) seeks to develop and test fast radiative transfer (RT) models and their adjoints in preparation for the direct assimilation of microwave radiance satellite data (current and future) under all weather conditions, particularly precipitating clouds, into NCEP’s Global Data Analysis System (GDAS). Scattering becomes more important for precipitation-sized particles at higher microwave frequencies, therefore, all models account for multiple scattering. A RT model based on the method of successive order of scattering is developed and compared with other fast models under a wide range of conditions. Results will be reported at the conference.