BECAUSE OF ITS UNIQUE COMBINATION of physical, chemical, and thermodynamic properties, water exists in the solid, liquid, and vapor phases over the normal temperature range of human existence and comfort. Each of these forms, either separately or in combination, can adversely affect or influence the performance and behavior of materials used for buildings. This chapter discusses the effects of moisture on the properties and behavior of constructional and functional materials except for thermal insulations and wood and wood products, which are discussed in Chapters 4 and 5, respectively.