Dew, a key component of Non-Rainfall Water Inputs (NRWIs), plays a disproportionately significant role in land-atmosphere interactions. This review synthesizes advances in understanding its ecological, hydrological, and environmental effects, quantification methods, and spatiotemporal variations. A key finding is the regional dichotomy of dew effects: in arid regions, it is a crucial hydrological source, whereas in humid/alpine regions, its energy-balance regulation via latent heat release often outweighs its hydrological contribution. Significant challenges persist, including methodological inconsistencies, an overreliance on point-scale data from arid zones, and an underappreciation of dew's energy impacts, particularly in cold regions. Recent studies suggest a general declining trend in dew frequency and amount in many arid regions, which could exacerbate water stress for dependent ecosystems. However, regional heterogeneities and interactions with other NRWIs remain poorly constrained. Future research must overcome observational bottlenecks, deepen energy-water coupling studies, quantify climate change impacts, expand research to underrepresented regions, and integrate multi-method approaches to improve model predictability, thereby supporting ecosystem resilience and water security under global change.