The primary function of the TRMM Ground Validation (GV) Program at NASA Goddard Space Flight Center (GSFC) is to provide ground-based surface rainfall estimates for validating satellite-derived precipitation retrievals from TRMM.  Data quality, retrieval techniques, and processing methods are significant elements of TRMM GV rainfall estimation.  A description of the data quality control effort, together with the science and processing methodologies employed at NASA-GSFC in the generation of reliable surface rainfall estimates will be presented.  Quality control procedures are first performed on radar and rain gauge data independently, then again in a combined method in the development of monthly Probability Matching Method (PMM) Ze-R lookup tables.  Monthly rainfall estimates have the additional concern of integration and data gap accountability.  Validation statistics (dependent and independent) from the current version (5.0) of GV rainfall validation products show marked improvement over previous estimates.  The challenges of estimating and validating surface rainfall within the context of the TRMM GV Program and applications to future precipitation missions will be discussed.