This chapter presents an overview of the major components of a vadose zone water budget or what soil scientists refer to as a soil-water budget. It sets the stage for more detailed discussions of some of the soil-water budget components that are most important in hydrogeologic investigations. The general equation for the soil-water budget is derived by considering the mechanisms by which water can enter, exit, or be stored in a predefined region of the vadose zone. Infiltration is the volume of water that crosses the land surface to enter the vadose zone. It is usually quantified in one of three ways: by the residual from a surface water budget analysis, by field measurements, or by calculation based on soil hydraulic properties. Evaporation refers to the water lost from the vadose zone by vapor-phase transport from the soil directly to the atmosphere. Transpiration is the water depleted from the vadose zone by plant root uptake and released to the atmosphere.