Cluster ion beam processes which employ ions comprised of a few hundred to several thousand atoms have been extensively developed during the 30 years since the concept originated. Low energy surface interaction effects, lateral sputtering phenomena and high-rate chemical reaction effects have been explored experimentally and have been explained by means of molecular dynamics (MD) modeling. Practical production equipment for a wide range of applications has also been successfully developed. The technology is now advancing rapidly in the fields of sub-nanoscale processing of metals, semiconductors and insulating materials. In this chapter, important historical milestones which influenced recent technological developments are reviewed by referencing their original contributions. Then, prospective new areas of applications are reviewed.