The basis for estimating water flux through forest stands is described using models. By means of the Penman-Monteith equation, transpiration is described in relation to the degree of coupling between the canopy and the surrounding air. Models of canopy and aerodynamic conductance are available, but more mechanistic models of stomatal conductance, and further development of turbulence theory, are needed along with improved estimates of leaf area index, leaf area distribution, and seasonal dynamics. Three models are presented to show current capabilities in estimating water uptake and flux through tree components, including the effects of capacitance. Defining conductance to water movement through tree components in terms of the properties of the pathway (sapwood area, sapwood relative conductivity, leaf area) is a useful functional approach that can be tested for a range of species at different sites. Further research is required to relate water conducting properties of tree components to architectural arrangement, especially for roots, and improved methods for measuring water potentials and partial flows at different points within the system are necessary. The role of water potential and the significance of its variability within a canopy are discussed. Relating growth processes to an integral of water potential (an accumulated product of water potential and time) is recommended. The need for scaling and integrating information about processes from one level to higher levels is recognized. The importance of understanding the roles that temporal, spatial, and developmental levels have on the ability to scale or integrate individual leaf measurements of, for example, leaf conductance to the stand level is emphasized.