The Microwave Integrated Retrieval System (MiRS) is a physically-based rainfall rate retrieval algorithm applicable to all microwave passive sensors that has been extended to include TRMM/TMI and GPM/GMI sensors. MiRS is an operational algorithm at NOAA/NESDIS and provides important hydrological information from POES AMSU/-MHS, and DMSP SSMI/S. The design of the algorithm is based on the generic One-Dimensional Variational (1DVAR) approach where the solution starts from a first guess and attempts to converge to a final estimate based on a forward model and its Jacobians. The MiRS algorithm retrieves surface emissivity, skin temperature and hydrometeors parameters, all at the same time, ensuring that the final solution is consistent with the measurements. This feature allows it to account for emissivity variations when obtaining rain estimates which is critical for rain retrieval over land using low frequency channels. The MiRS algorithm is valid in opaque conditions brought on by heavy rain where the emissivity is irrelevant or in clear sky conditions, and cases in-between. The MiRS algorithm also accounts for mixed pixels where the footprint is heterogeneous from mixed precipitation conditions (clear/rainy), and mixed surface types (land/river and coasts). MiRS processing of TRMM TMI data is being run on a daily basis at 9 km resolution near the Southeast US and Gulf of Mexico region, and globally at 63 km resolution. GPM GMI simulated data are being generated using European Centre for Medium-Range Weather Forecasts (ECMWF) fields collocated with the TRMM TMI, and used to produce MIRS GMI products daily over the same region and at the same resolution. Recent improvements to the MiRS TMI retrievals include smoother transitions of rainfall rates along coasts, increased detection of light rain events, and greater convergence coverage in heavy precipitation. In our assessment of the MiRS rainfall rate retrievals we will present point-to-point comparisons to TMI rainfall rates, and CONUS comparisons to the National Weather Service's Stage IV Radar data the Climate Prediction Center's rain gauge analyses. We will also illustrate the very dynamic response of MiRS emissivity through time series of emissivity in restricted regions over rain events.