Forest lands provide the cleanest and the most dependable supply of water compared to all other land uses. This holds whether forests are young or old, pine or hardwood, and managed or unmanaged. The connection between forests and clean water has long been recognized and decades of watershed science provides a solid understanding of the various processes that interact with rainfall as it falls on the top of the forest, drips down through the leaves, enters the forest floor, and is eventually used by the trees or flows into streams or recharges groundwater. Using examples from long-term watershed experiments, I will explain the basic hydrologic processes that occur as water moves through a forest. Topics to be covered include identification of critical forest watershed components (e.g., riparian zones) and discussion of the impacts of disturbances such as fire, insects, and management activities on water resources.