Nature-based solutions (NBS) restore green spaces and natural hydrological cycle reducing runoff generation, promoting infiltration or exfiltration and evapotranspiration (ET). ET governs much of hydrological and microclimatic functioning of urban NBS, yet it remains inconsistently measured, modelled and reported accross the literature. This review critically evaluates ET measurement and modelling in green roofs (GR), while positioning them within the broader urban NBS context. This structured narrative review synthesizes 104 studies published between 1948 and 2026, identified through database search and complementary reference-based search, critically evaluate ET modelling approaches, observational evidence, data requirements, and uncertainty implications in urban NBS. Based on this review, priority research needs include long-term multi-site observational datasets, standardised ET measurement protocols, and climate-adaptive modelling frameworks capable of supporting reliable NBS design and performance assessment under future climatic conditions.