THE INTENT OF THIS CHAPTER is to provide the practicing building designer with the minimum basic understanding of the hygrothermal phenomena, including moisture movement and transfer, and of its effect on building constructions, necessary to understand the chapters directly related to moisture analysis. It is not the intent of this chapter to replace more detailed handbooks or to supply a complete knowledge of building physics. Readers who wish to study the fundamentals of moisture thermodynamics in greater depth are referred to such excellent publications as the ASHRAE Handbook of Fundamentals [1], ASTM MNL 18, Moisture Control in Buildings [2], and others cited as references.