VizieR (http://vizier.u-strasbg.fr) groups in a unique database a large number of catalogues and tabular material having an interest for astronomical studies (over 15,000 tables). VizieR contains a large variety of heterogeneous data, including atomic data, observational and simulated data, logs of satellite missions, etc.
Retrieving data in this vast amount required a careful definition of the metadata associated to each column of each table (over 200,000 columns!), and their assignement to each piece of data. This exercise started a couple of years before the emergence of the Virtual Observatory, with trials of automated assignation of metadata from the data description. The metadata definitions and their assignation evolved in connection with the Virtual Observatory developments, and are now part of the vizier schema.
The history and evolution of the metadata in VizieR, and how these metadata are practically assigned, will be presented; the limits of the system, in terms of metadata accuracy and objectivity, will be discussed.