Wee felt it before in sense; but now wee know it by science. Edward Misselden (1623) The collective effort reported in this volume is the outcome of the diffusion of the idea of diffusion as a fundamental process in society. The considerable number of disciplines represented here indicates the weight of the problem area. The editors are to be congratulated for their initiative in drawing together present thinking at a vivid meeting, now also in print. An old­ timer in the business has not much to add. But maybe some things, bearing in mind that a Preface is a celebration and not a review. As always with ideas it is hard to identify those who first gave shape to the idea of diffusion. In a general sense it is probably an observation as old as human self-reflection that groups of populations exchange ideas and copy habits and implements from each other. Sometimes it has even been recommended, as a Chinese proverb suggested millenia ago, "If you want to become a good farmer, look at your neighbor" .