[en] The aim of the work described here is to develop a set of related mathematical models for the transport of radionuclides in flowing groundwater. These models have an important role to play in making safety assessments of geological disposal options, and in the planning and interpretation of experiments in both laboratories and the field. Since the announcement of NIREX preferred sites the priority is to have models which apply to real situations and are properly validated. The particular features of the programme are as follows: First, we are developing very general multidimensional models for flow and transport which can include many of the complications found in real rock masses, such as the presence of large fracture zones. Secondly, we are developing simpler but more efficient models forming approximate scoping calculations and sensitivity analysis. Finally, we are attempting to validate these models in all possible ways - by comparing with other models and testing against field and laboratory experiments. (author)