This paper reports on the findings of a recent IAEA expert meeting on the assessment of greenhouse gas (GHG) emissions from the full ＇lifecycle＇ of hydropower. It discusses the different categories of hydropower plants in view of the two main sources of GHG emissions: first, direct and indirect emissions associated with the construction of the plants; second, emissions from decaying biomass from land flooded by hydro reservoirs. In terms of GHG emissions, this paper shows that, in most cases, hydropower is a good alternative to fossil fuelled power generation. For hydropower plants in cold climate, a typical GHG emission factor is 15 g CO2 equivalent/kWh, which is 30-60 times less than the factors of usual fossil fuel generation. For some hydropower plants in tropical climates, theoretical calculations have shown that reservoir emissions could be very high. However, no measurements of emissions were taken from tropical reservoirs and the current level of research does not allow for a reliable evaluation. Research is urgently needed in humid tropical climates.