Operational analyses are affected by major changes in models, analysis technique, assimilation, and observation usage which are an essential product of research and progress. They also can make use only of those observations which become available within near real time. The ECMWF (European Centre for Medium-Range Weather Forecasts) performed a consistent re-analysis of atmospheric data using a "frozen" production system. The ECMWF Re-Analysis (ERA) Project produced a new, validated 15 year data set of assimilated data for the period 12/1978-02/1994. The assimilation scheme consisted mainly of the Integrated Forecast System (IFS) version of the ECMWF forecast model with T106 resolution on 31 vertical hybrid levels. An intermittent statistical (optimum interpolation) analysis with 6 hour cycling was performed. It includes a diabatic, non-linear normal mode initialisation (5 vertical modes). The data sets consist of multi-level, multi-code pressure fields which are stored in spherical harmonics in 6h-intervals (0,6,12,18).