In 2007, the Belgian Federal Authorities issued a Royal Decree concerning “inland vessels that can also be utilised for non-international sea voyages”, allowing inland vessels to operate in coastal areas between the Belgian coastal harbours and the Belgian inland waterway network via the Western Scheldt. These new regulations have a unique character due to their probabilistic approach: among other requirements, a risk analysis must demonstrate that the probability of adverse events such as bottom slamming, overtaking of water on deck and ingress of water in open cargo holds is limited to an acceptable level. Several inland vessels (tankers, container vessels, car carriers) are nowadays operating according to this Decree in significant wave heights up to 1.90 m. Background is provided into these present regulations, with emphasis on the probabilistic aspects. A description is given of the methodology used for performing the required risk analyses. The effect of important parameters is discussed, both ship related (metacentric height, bilge keels) and wave climate dependent, and an overview of the ongoing research at Flanders Hydraulics Research and Ghent University on estuary container vessels is given.