Catalysis is a subject which draws upon many of the traditional scientific and technological disciplines, and its present structure has only been reached after many decades of detailed work. Nevertheless, in catalysis as in all other areas of science, experiments are carried out, new data are acquired, theories are proposed; and these things are done not in an intellectual vacuum, but in relation to previously established concepts and ideas. The history of how these guiding concepts have devel oped is nothing less than a skeletal history of the subject itself. In one respect at least, catalysis is similar to all other of mankind’s endeavors: a failure to learn from history is a rejection of one’s heritage. None of us should willingly plead guilty to this, and Professor O.-M. Schwab’s opening chapter in this volume provides us with a ready means to avoid the need to re-invent what has been known for some time. Heterogeneous catalysis is dominated by the concept of a reactant molecule entering into some type of che mical interaction with the surface of a solid. Thus, to understand catalysis it is essential to understand as fully as possible the chemical nature of the solid. In the second chapter of this volume, Professor J. Haber provides a review of the majority of solid phases which are of catalytic interest. The framework for this review is chemical crystallography, and the author goes to some pains to draw attention to features that are of particular importance in catalysis.