Cirrus (ice crystal) clouds are predominantly composed of non-spherical ice crystals of varying shapes and sizes and as a consequence have an important impact on the earth-atmosphere radiation balance. Any changes to their microphysical characteristics will have a substantial feedback on climate change; hence it is vital to accurately represent cirrus cloud and their single-scattering properties in climate models and in remote sensing applications. In this paper the current understanding of the single-scattering properties of cirrus cloud at solar and thermal wavelengths is reviewed. (C) 2004 Elsevier Ltd. All rights reserved.