This paper describes significant changes to the U.S. National Centers for Environmental Prediction (NCEP, formerly NMC, the National Meteorological Center) global data assimilation system that were made operational in January, 1995. The emphasis is on changes to the 3D-variational analysis, which has been running operationally since June, 1991. The changes include additions of new data types, modifications to the background error, and addition of a weak constraint on divergence tendency. A uniform improvement in fit of the 6 hour forecast guess to all observation types was observed in parallel tests over a period of 9 months. Most of the improvement is believed to be due to the addition of the divergence tendency constraint. The improved performance extends to medium range forecasts, as measured by anomaly correlation scores for geopotential height. Current research activities related to global and regional data assimilation at NCEP are also briefly described.