In this chapter, we discuss the basic interactions of plasma-generated ions and charge-neutral species with solid and liquid surfaces. After presenting some phenomena caused by energetic ion impact on a solid surface, including collision cascades, we discuss surface chemical reactions and emphasize their importance in surface modification by plasmas. On a solid material surface subject to the irradiation of energetic ions and chemically reactive charge-neutral species, chemical reactions take place mostly in a thin layer of about one to a few nm thickness, which is called a mixed layer as the incident species are well mixed with surface atoms there. For soft materials such as liquid water, where diffusions of atoms are much faster than those in solids, most chemical reactions take place in a thin layer of about one to a few \(\mu \) m thickness, which is called a reaction boundary layer. For typical low-pressure low-temperature plasmas, the ionization rates are low, so the fluxes of charge-neutral species are much higher than those of ions. Therefore, free radicals generated in plasmas often play the dominant roles in surface chemical reactions. Especially, for reactive ion etching, free radicals significantly contribute to the synergy effects between surface chemistry and ion bombardment. For plasma-liquid surface interactions, plasmas are typically generated at atmospheric pressure or at a relatively high pressure where the liquid can exist stably. The charge-neutral fluxes are even larger than the ion fluxes for such plasmas, so the synergy effects between ions and surface chemistry hardly exist. In general, the reactions are driven thermally for plasma-liquid surface interactions.