About 1200 galactic open clusters are known and approximately half of them have been observed so far, in at least one photometric system. The number of stars per cluster goes from a few tens for the poorest objects, to several thousands for the most prominent clusters.
Modern observations of open clusters developed very rapidly after the definition of the UBV photoelectric system (Johnson &Morgan 1953). These observations produced a number of colour-magnitude (Hertzsprung-Russell) diagrams which made fundamental contributions to the understanding of stellar evolution. At the same time photographic photometry allowed to observe larger areas and reach fainter stars. Additional information, mostly spectroscopic, was gradually obtained, first for stars in the nearby clusters and later in more distant clusters, thanks to the existence of larger telescopes and more efficient detectors. Two-dimensional detectors are best adapted for the observations of star clusters and CCD observing is today becoming the preferred technique. It replaces both the photoelectric and photographic photometry.
Data compilations started already in 1972 at the Institute for Astronomy (University of Lausanne, Switzerland). Mermilliod (1976a) published a first catalogue of UBV photometry and MK spectral types in open clusters. The third version was announced ten years later (Mermilliod 1986a). The systematic determination of cross-references between the many numbering systems in a cluster was the basic work which made the realisation of the data collections possible. Several catalogues were distributed by the Strasbourg Data Center (CDS). The files remained on magnetic tapes until the installation of Unix workstations and large disks in our institute made it possible to keep the data on-line. These compilations were discontinued in their older form and the data were organised in a database designed in March 1987 (Mermilliod 1988a, 1988b).