This document provides an overview of key concepts in watershed hydrology. It defines hydrology as the science concerned with the origin, circulation, distribution and properties of water on Earth. Forest hydrology specifically deals with the effects of land management and vegetation on water quantity, quality and timing. The document emphasizes that a watershed approach to management is important because all areas within a watershed are connected by water flow. Managing activities and land use at the watershed scale can help avoid negative downstream impacts on water resources.