The historical Ypresian stage concept or Ypresian Synthem, on which this paper focuses, was introduced by A. Dumont in 1849 (minutes of the meeting published in 1850) to specify marine clays and overlying glauconitic sands occurring in the Ieper area (W Belgium), although without mentioning stratotype or type locality. This concept, which clearly evolved in the course of the 19th and 20th century, fundamentally differs from the GSSP-defined Standard Global Ypresian Stage (GSSP ratified in 2003 at Dababiya, Egypt). The latter ranges from 55.8 (± 0.2) to 48.6 (± 0.2) Ma and represents the lowermost Eocene Standard Stage. Here, we briefly discuss the different geological aspects of the Ypresian sensu Dumont, currently equated with the Ieper Group, including its stratigraphy and its sedimentological and palaeontological characteristics. The structural context and palaeogeography of the Belgian Basin during the Ypresian are outlined. The major scientific contributions are thematically listed.