The carbon-based materials and their response to ion-implantation and thermal treatments, which have been reviewed here, are unique in many aspects. The basic carbon materials considered, diamond and graphite, have many differences. Diamond exhibits 3-dimensional sp 3 tight tetrahedral bonding, and unusual physical properties result from this bonding (hardness, high thermal conductivity, optical transparency, high index of refraction and wide band-gap). Graphite exhibits a sp 2 bonded 2-dimensional layered structure which leads to highly anisotropic nearly metallic properties. These differences in bonding make the properties of these two allotropie forms of carbon about as different as one can imagine. Yet, as shown in the preceding chapters, ion implantation can cause non-reversible changes from diamond to graphite under conditions which are now fairly well understood.