The modelling of microstructure, and hence the properties, of cast alloys has so far been attempted assuming the liquid metal is free from defects. The growing appreciation that bifilms, of size measured in millimeters, are usually abundant in liquid metals, and control many features of the casting, has led to this paper in which some of the known and some of the expected effects are reviewed. It is suggested that bifilms cause major effects on microstructure and properties and their presence can vastly overpower the effect of traditionally simulated structural features.