Windbreaks have been promoted as a means of arresting degradation of agricultural lands in the Sahel resulting from deforestation, but competition for water between windbreak trees and adjacent crops must be minimised if windbreaks are to be successfully established in areas where crop production is threatened by loss of trees. Research was done, consequently, between 1991 and 1993 in southern Niger, in West Africa, to aid development of strategies for the deployment of windbreaks in the Sahel which minimise competition for water. The objectives of this research were to: (1) quantify water use by windbreak trees; (2) determine the source of water utilised by windbreaks; and (3) identify the environmental and physiological variables controlling transpiration by windbreak trees. The heat-pulse method was used to measure water use during the cropping season by three tree species growing in windbreaks and patterns of water extraction from soil adjacent to windbreaks of each species were assessed from soil moisture contents measured by neutron attenuation. Azadirachta indica transpired less water than either Acacia holosericea or Acacia nilotica and extracted less water from the rooting zone of nearby crops, and was thus shown to be the least competitive of these tree species. Naturally-occurring variations in the ratio of the stable isotopes of oxygen (¹⁸O/¹⁶O) in water were utilised to trace the sources of water transpired by Azadirachta indica trees in windbreaks and adjacent crops of pearl millet (Pennisetum glaucum) at two locations with contrasting water table levels. Where groundwater could be accessed by the trees, they obtained large proportions of their water from surface layers of the soil only when water there was plentiful, for example after rain. Where groundwater was not accessible, the trees fulfilled their requirements for water from the surface layers of the soil profile throughout the year. Thus, competition for water between windbreaks and crops is more severe at locations where trees cannot utilise groundwater.