Water is one of the key deterioration factors for porous building materials and has the capability of enhancing the deleterious effect of other factors such as air pollution and biocolonization, among others. The focus of the paper is the interaction of water with the porous inorganic materials: how does water enter the pore system, and how does it leave it? How does the presence of clays or soluble salts in porous materials affect these mechanisms? Pairwise examples are provided to illustrate the varying behavior of water in different materials. Also addressed is the importance of the pore size distribution on the behavior of water and its transport mechanisms. The aim of the paper is to synthesize the key points required to understand why water enters a porous system, in what way it is distributed within it, and the slow manner in which it departs, since time of wetness is fundamental in enhancing the effect of co-factors.