This paper reviews the literature on moisture barrier properties of polymer/clay and polymer/graphene-based nanocomposites. The various models proposed to predict the effects of nanofillers in reducing water vapour permeability through polymers are outlined. These models are based on a range of different factors such as; tortuosity, geometry, platelet stacking, orientation, polymer chain confinement and plasticization. Published experimental studies of water vapour permeability in both polymer/clay and polymer/graphene nanocomposites are then reviewed. The extent to which the models are validated by the results of these studies is discussed, together with the degree to which the potential for water barrier improvement in polymer nanocomposites has been realised. •Description of models predicting water permeability of polymer nanocomposites.•Introduction to polymer/clay and polymer/graphene nanocomposites.•Review of published studies on water barrier properties of polymer nanocomposites.•Discussion of validation of models.•Moisture barrier applications for polymer/clay and polymer/graphene nanocomposites.