The book demonstrates the power of skeletal numbers to enable the classification of clusters of the main group elements and transition metals into clans and families. The skeletal numbers are utilized to decompose a chemical formula into a whole number, from which the series formula S=4n+q and the categorization parameter K = Cy+Dz are readily derived. Both formulas can be applied to any cluster formula to calculate the cluster valence electrons.This book provides more than 60 cluster examples worked out and most of them explained. The range of clusters include, metal carbonyls, boranes, metalloboranes, carboranes, golden clusters, zintl ions, matryoshka clusters, and giant clusters. The book also demonstrates the fact that skeletal numbers and cluster valency electrons of main group and transitional metal elements are intertwined.