Attempts to explain observations of catalytic selectivity in highly ordered materials with different pore dimensions and different degrees of pore interconnectedness have driven the general theoretical development of shape selectivity in molecular sieves. Fresh theories have emerged with the discovery of new molecular sieves, or as newly found catalytic importance has shifted attention back onto older materials. Highly evolved stochastic and molecular dynamic models are able to accurately represent the primary effects of shape selectivity, yet gaps in our overall understanding still remain. The fundamental concepts underlying shape selectivity have been used very effectively in the development of new catalysts for the petroleum and petrochemical industries. While the focus has recently moved to the more extensive application of shape-selective catalysis to fine chemicals, significant advances in the form of new processes and applications of new materials continue to be made in petroleum refining and commodity petrochemical industries. As the concept of shape-selective catalysis in molecular sieves nears its forty-third anniversary, several of its theoretical bases remain topics of intense study. (C) 2003 Elsevier Science (USA). All rights reserved.