Tropical forests are responsible for the evaporation and transpiration of large quantities of water into the atmosphere annually. Surface conductance (g(s)) is a poorly understood phenomenon that plays a central role in regulating this evapotranspiration. We studied the calculations, variations, and environmental factors controlling g(s) based on eddy flux measurements from 10 tropical forest sites that covered a wide range of water gradients across continents. We found that boundary layer conductance (g(b)) is comparable with aerodynamic conductance for momentum (g(aM)) and thus, it should not be ignored in estimations of total aerodynamic conductance for water vapor (g(aV)). Based on the findings, we have made some recommendations for g(aM) estimation both with and without measurements of turbulence. The seasonal variation of g(s) is low in humid sites but large in sites with a dry season. A value of 24.8 +/- 13.8 mm s(-1) was suggested for maximum surface conductance (g(smax)) for tropical forests. Both water vapor deficit (D) and radiation (Q) play an important role in controlling g(s). The model driven by both D and Q could capture the diurnal variations of g(s) well and could be implemented in large-scale models in future. We believe the findings of this study could contribute substantially to our understanding of tropical forest g(s).