It has been 15 years now since the idea of the gas cluster ion beam (GCIB) process first came up, but the interest in GCIB process has increased only recently, driven by the nano-technology program, especially in Japan. This ion beam process uses a beam of ions consisting of clusters of a few hundreds to thousands of atoms generated from gaseous materials. The impact of these accelerated cluster ions with the surface produces a low energy bombardment at very high density. The cluster-surface collisions were found non-linear effects and to have unique characteristics which were found to be useful for applications in novel surface processing. These characteristics include surface effects such as shallow implantation, lateral sputtering, cleaning and smoothing, as well as low temperature thin film formation. This paper reviews the current fundamental research related to the GCIB-surface interactions as well as their applications in modern magnetic, optical and semiconductor device fabrications. These presently include: (i). IC Back End of the Line (BEOL) materials processing, (ii). surface smoothing of metals, dielectrics, superconductors, and diamond films for optical and magnetic devices, (iii). selective smoothing of SOI, SiC and compound semiconductor films and (iv). high quality thin multi-layer film deposition for reliable and durable optical filters.