An improved understanding of the basic interactions between radiation and the atmosphere underpins practically all research in atmospheric radiation and serves as the building block upon which all application research is based. For example, an intrinsic understanding of the interactions that occur on the molecular scale is fundamental to understanding radiative forcing of climate induced by global changes in the concentrations of greenhouse gases. Advances in our understanding of particle scattering, for instance, is also crucial for understanding how clouds affect radiation and is the basis not only to studies of cloud--climate issues as described below but also provides the physical rationale for most of the remote sensing techniques presently used to study clouds.