Fire dramatically alters hydrologic processes in many regions of the world. Individual fires reduce vegetation and change soil characteristics, sometimes producing dramatic runoff events in the years shortly after a fire. The greatest determinant of the effect of fire on runoff generation is the severity of the fire, which relates to the frequency of fires and other climatic and vegetation characteristics. Severe fires can produce hydrophobic soils or increase risk of soil surface sealing, reducing infiltration rates. Measurements of the spatial pattern of water repellent soils are useful for estimating potential runoff from postfire storms. The most severe events occur during convective storms, so the spatial extent of individual postfire floods is generally limited in extent. Recovery of water repellent soils is relatively rapid, with significant reductions occurring within a few years. Longer-term changes to hydrology are related to the reduced evapotranspiration caused by loss of vegetation biomass. In forests, changes to annual water balances may last decades.