A profusion of research and results on the mechanical behaviour of crystalline solids has followed the discovery of dislocations in the early thirties. This trend has been enhanced by the development of powerful experimental techniques. particularly X ray diffraction. transmission and scanning electron microscopy. microanalysis. The technological advancement has given rise to the study of various and complex materials. not to speak of those recently invented. whose mechanical properties need to be mastered. either for their lise as structural materials. or more simply for detenllining their fonnability processes. As is often the case this fast growth has been diverted both by the burial of early fundamental results which are rediscovered more or less accurately. and by the too fast publication of inaccurate results. which propagate widely. and are accepted without criticism. Examples of these statements abound. and will not be quoted here for the sake of dispassionateness. Understanding the mechanical properties of materials implies the use of various experimental techniques. combined with a good theoretical knowledge of elasticity. thermodynamics and solid state physics. The recent development of various computer techniques (simulation. ab initio calculations) has added to the difficulty of gathering the experimental information. and mastering the theoretical understanding. No laboratory is equipped with all the possible experimental settings. almost no scientist masters all this theoretical kno\vledge. Therefore. cooperation between scientists is needed more than even before.