As a boy I loved to build model airplanes, not the snap-together plastic models of today, but the old-fashioned Spads and Sopwith Camels made of balsa wood and tissue paper. I dreamed of EDDIE RICKENBACKER and dogfights with the Red Baron as I sat there sniffing airplane glue. Mother thought I would never grow up to make an honest living, and mothers are never wrong. Thirty years later I sit in a research laboratory surrounded by crystal models and dream of what it would be like to be 1 A tall, to rearrange atoms with pick and shovel, and make funny things happen inside. Professor VON HIPPEL calls it "Molecular Engineering," the building of materials and devices to order: We begin to design materials with prescribed properties, to under­ stand the molecular causes of their failings, to build into them safe­ guards against such failure, and to arrive at true yardsticks of ultimate performance. No longer shackled to presently available materials, we are free to dream and find answers to unprecedented challenges. It is this revolutionary situation which makes scientists and engineers true allies in a great adventure of the human mind [1]. This book is about structure-property relationships, more especially applications of crystal chemistry to engineering problems. Faced with the task of finding new materials, the crystallographer uses ionic radii, crystal fields, anisotropic atomic groupings, and symmetry arguments as criteria in the materials selection process.