The ratio of transpiration to evapotranspiration (T/ET) is a key parameter to illustrate the role of vegetation in controlling water loss in terrestrial systems. Quantifying evapotranspiration and its components is a key to understand the dynamics and mechanism of evapotranspiration and it is the basis for studying the eco-hydrological process. This paper reviews the research progress of T/ET quantification methods (measurement methods and model methods), T/ET results in different ecosystems and their driving mechanisms (climate change and vegetation cover). The methods are introduced in T/ET observation and simulation developed at home and aboard, and the results of different methods are compared. The values of T/ET are described in four types of terrestrial ecosystems, including forest, grassland, wetland and farmland. There are significant differences in T/ET in the same or even in the different ecosystems, which are mainly due to the differences in ecosystem types, spatial and temporal scales, selection of data set, observation and simulation methods. The main driving mechanisms of T/ET included vegetation type, vegetation cover, climate factors and soil factors. In the future, uncertainty studies around different quantification methods of T/ET, changing patterns and driving mechanisms of T/ET in different terrestrial ecosystems will be emphasized.