The necessity and difficulty of marrying together micro and macroscopic aspects in problems of plasticity are stressed and analysed on some examples. The importance of a mesoscopic scale is suggested. Introduction. When I was writing in 1953-4 my book on Dislocations, my friend B. Jaoul was composing, in the same lab., his own contribution to the mechanics of plasticity. It seemed to us at the time, and I still strongly believe, that the micro and macroscopic points of view from which these two

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... books were starting should be ideally joined together. This is indeed the spirit in which this special issue of the Journal of the Mechanical Behaviour of Materials has been composed. But the road towards this goal has been especially bumpy ; and we are still far from a perfect matching of these two approaches. This is what I want to stress, taking a few important examples :plastic deformation of pure metals, plastic relaxation of cracks, strain localisation in alloys. I -Plastic deformation of pure metals. Here, the microscopic description starts from a network of dislocations, either a simple Frank network or a polygonised mosa'ic structure, purporting to describe a well recrystallised single crystal. When the stress is applied at moderate temperatures, the most mobile dislocations under the largest resolved shear stress start acting as Frank Read sources, and produce the first slip bands.