Silasesquioxanes are an interesting class of ligands for both main group and transition-metal elements. A variety of coordination environments can be supported by silasesquioxane ligands, and it is now possible to prepare metallasilsesquioxanes containing practically any stable element from Groups 1015. This article surveys the synthesis, characterization, and reactivity of silasesquioxane ligands. Structure, bonding and structural dynamics of both silasesquioxanes and metallasilasesquioxanes are also discussed. The chemistry of silasesquioxanes and metallasilasesquioxanes can provide important insights into the chemistry of silica and silica-supported transition-metal catalysts. The uniqueness of silasesquioxanes as models for silica is discussed and several systems which provide molecular-level insights into the surface chemistry of silica and silica-supported catalysts are discussed.