Moisture, when unmanaged, can be the source of a variety of problems affecting the life of both the interior and exterior of swine facilities. It can cause rot, corrosion, mould, staining, shrinkage and/or swelling, and insect infestations. Moisture balance is based on wetting and drying. Sources of wetting include rain, air, built-in moisture, and soil moisture. Sources of drying include drainage, evaporation, air leakage, and heating. The amount of water vapour depends on the air’s capacity to hold it and is affected by temperature. Condensation occurs when the maximum holding capacity is exceeded (warm air can hold more). Water vapour moves by diffusion (more moisture to less moisture) and convection (high pressure to low pressure). Sources of moisture include livestock (manure, urine, and respiration), equipment (drinkers, pipes, heaters), and structure (water “ponding”, ice damming, air leaks, etc.). Moisture problems specific to swine facilities include corrosion, mould, and decay. Control of moisture is important and can be done by rain and air leakage control, drying, and adequate ventilation. Manure and urine should be removed as quickly as possible. Equipment should be maintained and repaired so it functions efficiently, and structures should be designed properly to minimize problems that may occur.