Molecular-level understanding of the chemical processes involved in water adsorption on metal and other surfaces is important in the fields of wetting, corrosion, catalysis, and electrochemistry. Here I would like to present a series of studies on the adsorption and reactions of water on metal and graphene surfaces using scanning tunneling microscopy (STM). At low coverage, water adsorbs on Pd(111) surface in the form of isolated molecules, which diffuse by hopping to nearest neighbor sites. Upon collision, they form water clusters (dimers, trimers, tetramers and so on), of which the mobility is increased by several orders of magnitude than monomers. As coverage increases, water film growth starts by the formation of flat hexamer clusters and then larger networks of linked hexagons with a honeycomb structure, following a two-dimensional (2D) ice rule. Water adsorption on epitaxial graphene on Ru(0001) substrate was also studied. We found that the graphene line defects are extremely fragile towards chemical attack by water, which splits the graphene into numerous fragments at temperatures as low as 90 K, followed by water intercalation under the graphene. This process resulted in many graphene flakes that were displaced onto the first graphene layer on Ru. The superlubric sliding behavior and electronic screening effect of these flakes were also investigated.