The relationship between water and ecosystems, an important hotspot in the scientific fields, is highly concerned by the whole human society. Water retention and hydrological regulation are both key services produced by terrestrial ecosystems, which closely link the health and integrity of ecosystems and sustainable socioeconomic development that calls for dynamic and comprehensive perspectives for advancing the research paradigm. This paper firstly clarified the scientific concepts of water retention and hydrological regulation based on the principles of eco-hydrological processes. Water retention is the capacity of ecosystems to hold part of water input from precipitation at certain spatiotemporal scales, whereas, hydrological regulation is the capacity of ecosystems influencing the hydrological cycles across space and time. In fact, water retention is an integral component of hydrological regulation. The spatiotemporal variation of water retention can have important impacts on the roles of ecosystems in hydrological regulation services. The former is specific and should be used accurately, while the latter is more inclusive, objective, and widely usable. Then, a brief analysis on the assessment methods of the two hydrological ecosystem services was given in the paper. It was found that water storage estimation at different spatial scales was the staple method for the assessment of water retention service, whereas, water balance partition based on precipitation and evapotranspiration and complex models (e.g., SWAT) were most widely used for hydrological regulation service assessment. Anyway, spatial scaling and temporal dynamics should be incorporated in ecohydrological monitoring, assessment, and modeling, which is a weak point in contemporary research and applications. From the ecosystem services interaction point of view, there are complex trade-offs and dynamic synergies among water retention and hydrological regulation and other relevant ecosystem services (e.g., carbon sequestration, soil conservation, biomass production, and freshwater provision). More and more hydrological monitoring and experiments indicated that water retention and hydrological regulation by terrestrial ecosystems do not necessarily contribute to the fresh water provision to human society. Vegetation cover increment (e.g., from grass to forests) usually reduce the river runoff and thus the availability of water resources to human use from the surface water sources. This reduction is resulted at least partly from the increased water retention by changed ecosystem condition. Actually, the increase of water retained by ecosystems contributes to their services, such as carbon sequestration, soil conservation, biomass production, other than fresh water provision to river channel and human society. This is one of the important reasons for the trade-offs concerning ecosystem and water resources management. The tade-offs should be considered comprehensively in scientific research and ecosystem management applications, which guarantee accurate scientific understanding of the related issues and facilitating highly effective and sustainable development of the "water-ecological-social systems". As a prerequisite, long-term ecohydrological observations and experimental researches should be strengthened. Meanwhile, more attention should be paid to the spatial heterogeneity, scale effect, temporal dynamics, and the multi-dimensionality of the demands from various stakeholders pertinent to water resources and ecosystem management in integrative analyses. The advancement of the eco-hydrological understanding on hydrological services of ecosystems and related trade-offs and synergies are crucial for informing more rational and effective ecological restoration, conservation, and ecosystem management actions taken place on the ground.