The observation by Haines in 1923 that the decrease in volume of a clay soil during drying was at first equal to the volume of water lost is the cornerstone for the theoretical developments of recent years. Unfortunately the theoretical developments have been limited to uni-dimensional expansion and therefore do not relate directly to the complicated three dimensional field situation. Three have been many field observations of the behaviour of clay soils under a varying moisture regime but relatively few actual measurements. The work by Reginato (1976) and Blake et al. (1973) are examples of field measurements that have aided understanding of the phenomena, more investigations of these kinds are needed along with accurate determination of the soil parameters that are significant. The model developed by Hoogmoed and Bouma (1980) represents an additional step in developing prediction capability. Certainty there is ample opportunity for additional research.