It is 25 years since the seminal papers describing the relationships between the skeletal geometrics and closed shell electronic requirements of metal clusters were originally published. The electron counting rules which developed from these papers, are now to be found in all undergraduate inorganic textbooks. This volume provides a contemporary account of recent applications of the methodology and the mathematical foundations of the rules. In addition it describes how it is possible to accomondate acception to the rules within a wider framework. The authors are the foremost authorities in the field and give a broad introduction to the area.