This book is based on a series of lectures given by its author in Sweden, during March, 1948, at the invitation of Svenska Träforskningsinstitutet, Stockholm. The lectures, which were attended by industrial scientists from several of the Scandinavian countries, marked the inauguration by the Institute of what is hoped may develop into a general policy of inviting foreign scientists to lecture on specialized subjects connected with wood and paper. The lectures are essentially a descriptive account of the fundamental work done between 1940 and 1948 by the Physics Section of this Laboratory on the thermodynamics of the swelling of gels, and refer particularly to wood, wood fibres and paper. They outline the hypothesis of swelling stresses and the methods of calculating elastic constants of gels from swelling data (as more rigorously developed in the author's previous Special Report No. 6) and then go on to discuss the application of thermodynamics to the phenomenon of sorption hysteresis and to the stresses set up in plastic swelling materials, a category to which wood of course belongs. Swelling stress methods are important in dealing with porous bodies, where external mechanical behaviour gives no precise indication of the elastic and plastic properties of the constituent gel, and also in the study of stresses set up in glued members or in composite wood-resin structures, either during manufacture or when exposed to subsequent changes of humidity. It is hoped that the book will prove of interest and assistance to workers not only in the physics of timber but also in the wider field.