While much attention is given to warding off wind-driven rain, very little is given to interstitial condensation in masonry. Such condensation can lead to significant mould and mildew damage. Moisture movement through masonry walls is discussed, looking at the different effects obtained in winter and summer. The use of air barrier systems, which are designed to prevent the air from reaching a point in the wall cool enough to cause precipitation, is discussed. Air barrier membranes are available in sheet form in a variety of materials (liquid applied rubber copolymers, self-adhesive rubberised asphalt, EPDM and elastomeric bitumen). The air barrier system is a continuous and durable network of materials and joints providing air tightness with adequate strength and stiffness to prevent excessive deflection under air pressure differences. Placement, insulation of cavities, and building code requirements for air barriers are outlined.