Forest hydrology covers the water cycle in forests and tree-dominated landscapes. It describes the processes which lead to partitioning of rainfall water into water which returns to the atmosphere via evapotranspiration (green water) and water which contributes to the discharge in rivers and streams via groundwater flow and surface and subsurface flow (blue water). There is a tight link between water fluxes and biogeochemical processes controlling water quality, i.e., erosion and leaching of nutrients and contaminants. Distribution of green and blue water fluxes is determined by the climatic characteristics, topography soil properties, and vegetation. A number of human activities affect the water cycle in forest ecosystems: the most severe ones are deforestation and conversion to other land uses/cover like pasture, agricultural cropland, or urban areas. Upon conversion, factors controlling the hydrological processes are perturbed which leads to changed behavior of the blue and green water fluxes. Tropical forest ecosystems are determined by high input of energy and water which leads to unique processes in the water cycle. This chapter describes the major hydrological processes in tropical forests which control the conversion of rainfall into blue and green water fluxes. The consequences of human activities on the hydrological processes are also discussed.