Nanosized particles dispersed in water are part of vital processes that occur, for example, in aquatic eco-systems and in physiological media. Important applications of nanocrystals depend on their long-term stability in aqueous media and on the type of interactions with the surroundings, which are mediated by the established interfaces. Recent progress in the surface chemistry of nanocrystals dispersed in water is also due to knowledge accumulated from colloid and interface science, which has always had these systems as a central topic in its development, such as in the preparation of monodispersed colloids. This chapter describes aspects of the surface chemistry of nanocrystals dispersed in aqueous media, based on the fundamentals of colloidal science, although taking into consideration that nanoscale materials have peculiarities that make them distinct from conventional systems. In particular, this chapter reviews the formation of water-compatible inorganic nanocrystals having bare surfaces, and also those that are water stable due to the adsorption of foreign ions or to the chemical modification of the surface with hydrophilic species.