BUILDINGS COMPRISE A SIGNIFICANT PORTION of the assets of any nation. Hence, the structural integrity, durability, and performance of buildings play a vital role in the nation's economy. But buildings fail, and a major cause of failure are the thermal and moisture loads incorrectly accounted for during building design and construction. Building components and structures were developed through traditions and generations of experience, but far too often new building materials and construction practices are introduced without thorough analyses of their hygrothermal behavior. These analyses can be done through laboratory and field experiments and through calculations. While laboratory and field experiments are often too selective and rather slow, calculation methods can accommodate a variety of changing boundary conditions and result in much faster analysis. With rapid advances in computer technology and a better grip on numerical methods, many computer models for hygrothermal calculations were developed during the past decade. Depending upon the complexity of the problem under consideration, such models can be based on very simple, one-dimensional steady state methods or on very complex, two- or three-dimensional, transient methods. The general philosophy of modeling was presented in Chapter 1. This chapter briefly reviews the current state of modeling capabilities and presents selected applications of a computer model to illustrate the power ofmodeling as a tool for developing building design guidelines.