Nothing in our known universe is at once as constructive and destructive as water. It supports life while it turns stone to sand. It provides energy and delivers essential services to our built environment, while simultaneously degrading the materials in that built environment by corrosion, erosion, dissolution, decay, and expansion due to freezing. The papers presented in this Special Technical Publication (STP) explore both the problems associated with water in the exterior walls of buildings and the solutions to those problems.