Aerosols play an important role in climate and air quality. They have a direct effect on climate by scattering and/or absorbing the incoming solar radiation [Haywood and Boucher, 2000]. Reflection of solar radiation increases the atmospheric albedo, causing a negative radiative effect and therefore cooling of the atmosphere. On local scales absorbing aerosols can cause net positive radiative forcing resulting in warming of the atmospheric layer. Aerosols have an indirect effect on climate through their influence on cloud microphysical properties and, as a consequence, on cloud albedo and precipitation. The aerosol net effect on the Earth’s radiative balance depends on the aerosol chemical and physical properties, the surface albedo and the altitude of the aerosol layer [Torres et al., 1998]. The uncertainty in the effect of aerosols on climate stems from the large variability of aerosol sources, i.e., their concentrations and physical, chemical and optical properties, in combination with their short atmospheric residence time of a few days. In the IPCC (2007) [Forster et al., 2007] assessment report the total direct aerosol radiative forcing as derived from models and observations is estimated to be — 0.5 [ ± 0.4] Wm−2, with a medium-low level of scientific understanding. The radiative forcing due to the effect on cloud albedo is estimated as — 0.7 [— 1.1,+ 0.4] Wm−2 with a low level of understanding. Long-lived greenhouse gases are estimated to contribute + 2.63 [ ± 0.26] Wm−2. Improved satellite measurements have contributed to the increase of the level of scientific understanding since the third IPCC assessment report in 2001. The continued improvement of aerosol retrieval from satellite-based instruments, to provide consistent information with a known level of accuracy, is important to further our understanding of climate and climate change.