A very wide range of catalytic conversions find industrial use in organic process chemistry. The scale of the ope­ rations varies enormously from very high volume pro­ cesses to specialty chemical preparations. Many of these processes are functional group conversions or class reac­ tions, and the more important of these will receive detailed treatment in specific chapters throughout this series. Nevertheless, the scope is very broad, and it is all too easy for the non-specialist to become lost in a large volume of detail. To try to avoid this, the first chapter in this volume, by Dr. Paul N. Rylander provides a working summary of the more important catalytic con­ versions of this type. In doing this, he also gives some valuable comments about catalyst selection, together with an indication of the reaction conditions used in practice, the more important of the problems usually encountered, and comments about the most important of the mechanistic features. It has long been recognized that an understanding of the chemical nature of solid surfaces is fundamental to an understanding of catalytic processes which may take place upon them. This question may be approached in two distinct ways. One is via surface crystallography which focuses attention upon long range order. The second concentrates upon the concept of the surface functional group where attention is mainly upon the chemistry characteristic of a particular localized atomic arrangement at the surface. In practice, of course, there exists a continuum between these idealized extremes.