In a general way, "percolation" might be defined as a process in which at least two phases, one continuous and the other dispersed, are in bulk relative movement and exchange heat and/or mass through their interface. In practical terms this covers operations such as adsorption, ion exchange, leaching, washing, fluid-fluid displacement in po rous media, deep bed filtration, chromatography. Apart from the fact that the design of these processes is almost empirical we have concluded to the necessity of organizing a Summer School on this topic because 1. Powerful new concepts have emerged in this area in the last decade, centered around unsteady-state, non-linearly coupled, multi component systems, and low energy processes 2. There is, potentially, a phenomenologythat is common to all percolation operations, and which even extends to other mi gration phenomena such as electrophoresis, sedimentation, traffic flow 3. There is a need for a synthetic and didatic approach to these problems and its spreading will be most fruitful for the de velopment of separation science. We have tried to choose lecturers who have, themselves, ma de significant contributions toward the development of such an approach. The lectures given at the Summer School held at Espinho, Portugal in July 17-29, 1978 were compiled in this volume. It is VI divided in three parts. In the first part some general and intro ductory notions common to all operations are presented and chroma tography is analised in detail."