To understand the potential roles of seasonally dry tropical forests (tropical monsoon forests) of mainland Southeast Asia in the regional climate system, we briefly describe the complex forest types in this region and review published studies which reported water vapour exchange between tropical monsoon forests and the atmosphere. In addition, we propose future research needs to address deficiencies in our current knowledge base. Seasonal variation in atmospheric evaporative demand, which generally increases with atmospheric water vapour deficit and incoming solar radiation, is conspicuous in this monsoon region, reaching maxima in the late dry season and relatively low values in the wet season. Though the number of reviewed studies is limited, a comparison of seasonal patterns of evapotranspiration (ET) between forest types shows increased ET corresponding with the atmospheric evaporative demand in two evergreen forest ecosystems, but not in both an evergreen and two deciduous forest ecosystems. This contrasting ET seasonality results in considerable differences in dry season ET among forest types, ranging from a low of 0.6 mm d(-1) at one evergreen forest type, to 5.9 mm d(-1) at another evergreen forest type. Further investigations of both rooting-depths and responses of stomatal conductance of individual trees to dry conditions are suggested to be necessary to draw a more general conclusion regarding dry season ET difference between forest types. Besides the seasonal changes in evaporative demand, inter-annual fluctuations of rainfall and its seasonal distribution are also significant in this region. Recent phenological. and physiological studies suggest that these inter-annual variations in rainfall can have significant impacts on water and energy exchanges above the tropical monsoon forests. Thus, we stress the need for further examination of the effects of rainfall-regime anomalies on water and energy exchanges between tropical monsoon forests and the atmosphere. (C) 2008 Elsevier B.V. All rights reserved.