The present dune system probably originated in the 14th Century, but evidence of earlier dune activity in the area was found. The water table of the area has a convex surface and is not affected by tidal fluctuations landward of the coastal dunes. A quantitative basis for the definition of dune and slack levels, with ecologically meaningful limits, is suggested. Dune plant associes are defined as those where the free water table is always below 2 m from the soil surface. Dry slack associes are those where the free water table in summer lies between 1 and 2 m from the soil surface and wet slack associes those where the free water table never falls below 1 m from the surface. The range in pH value (4.4-8.5) of the soil at Newborough is greater than any so far recorded for sand dunes in the British Isles. Organic matter and free carbonate determinations are given for a range of dune and slack soils. The soils of the dune slacks are mainly base-saturated, but poor in potassium and phosphates. A brief account of the influence of man and animals on the development of the dune system is given. Soil forming processes are accelerated in the slacks compared with the dunes and sufficient organic matter accumulates in stabilized slacks to support an appreciable population of earthworms. The surrounding dunes have a big effect on slack development as a source of sand, nutrients and flood-water. Slacks, by providing essential conditions for the germination of Salix repens, which can then grow up through accreting sand to the dune level, indirectly modify the dune vegetation.