We have modelled the movement of groups of entities over realistic terrain using the equations of fluid dynamics. In our scheme, individual vehicles, people and even natural phenomena are modelled as localised packets of a fluid continuum that interact with a complex environment. The approach preserves the character of the individual and at the same time governs the long-range motion of the group. The scheme supports the co-ordination of large numbers of entities in a low-cost and plausible fashion. We provide details of how our technique has been exploited to represent the progress of crowds of people, and discuss in broad terms the scheme's general applicability to modelling aspects of Operations Other Than War (OOTW). We also demonstrate how established route planning techniques for single entities can be applied at the aggregate level, to guide the progress of groups of entities along roads and across open terrain.