Mudrock hydrogeology is important from the point of view of waste disposal, subsidence and aquifer analysis, and yet in Britain there has been little systematic study of the subject. Here a preliminary assessment is made of the concepts involved and the data already available. A wide variety of techniques can be used for measuring hydraulic properties of mudrocks, and often both laboratory and field determinations are necessary in order to gain an appreciation of the relative importance of the various possible flow mechanisms. It is uncertain to what extent the conclusions obtained by investigating ideal media are applicable to natural systems. However, research into natural systems elsewhere in the world indicates the potential importance to groundwater flow of lithification processes. The mudrocks of Britain can be divided into two groups, a pre-Mesozoic group, where fissure flow predominates, and a post-Palaeozoic group, where inter-particle flow may be present and where high rock compressibility may be important. As expected, both groups have extremely low hydraulic conductivities; the latter group is also characterized by moderate to high total porosities and by very high specific storage coefficients. Typical properties are presented.