The mechanisms involved in the transport of water in high polymers are reviewed. The discussion is mainly based on intact capillary-free models where the solution-diffusion mechanism is operative. The different patterns of behaviour which have been found for the sorption isotherms and the concentration dependence of the diffusion constants will be presented. Examples of the behaviour found with cellulose itself, two cellulose derivatives and with grafted cellulose and celullose acetate will be discussed in detail. In particular, experiments concerned with the sorption ofwater into wood pulp and grafted wood pulp will be presented and some theoretical and practical aspects of the results obtained considered.