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.