What is the magnitude of the Earth’s greenhouse effect? If by greenhouse effect we mean the downwelling thermal infrared radiation to the surface, many will be surprised to learn that values as low as 324 Wm-2 (4th IPCC report, 2007) and as high as 350 Wm-2 (recent assessments from observations) have being quoted in the last decade. This range of values is substantial, about ~10% of the solar energy absorbed by the planet (or emitted as thermal radiation to space). Why haven’t we yet pinned down such a fundamental quantity, and are we similarly uncertain about other components of the Earth’s energy budget? My presentation will attempt to shed some light on what we know relatively well and what we don’t know so well about the Earth’s energy budget, and on why radiative balances and imbalances are important for the Earth’s climate and should continue to be monitored.