Clay minerals are layered magnesium or aluminum silicates, which are abundant in the earth’s crust. Used since ancient times for the fabrication of bricks or terracotta, they now find application in the pharmaceutical and plastics industries. They play an essential role in oil and gas recovery, in water availability, and in preventing the dissemination of pollutants. In all of these contexts, the relevant properties of clay minerals are intimately linked to their microscopic structure, which results in a rich behavior with respect to water, solutes, and other fluids. This article provides a brief overview of the structure, dynamics, thermodynamics, and reactivity of water in clays, highlighting the role of the various types of water–mineral interfaces. Based on recent experimental and simulation studies, we discuss several features of these interfacial materials arising from their interactions with water on the molecular scale, including swelling, wetting, hydrodynamics in clay nanopores, reactivity of clay edge sites, ion exchange, and sorption.