Classical papers by Badon Ghijben and Herzberg, dealing with the saltwater–freshwater interface in an unconfined coastal aquifers—are presented in some detail first, including the historical context. What follows is a discussion of the conditions that have to be fulfilled for the Ghijben–Herzberg equilibrium (GHE) to be valid. Moreover, generalized formulations and findings, which have been developed during the century, are presented. It is shown that the GHE can be a useful rule of a thumb for confined aquifers, for general two‐phase fluid situations, for saltwater upconing below wells, and for situations without a sharp interface. The GHE can even be included successfully in computer models. Finally, a precursor of the classical papers is mentioned.