The interaction of clouds with solar and terrestrial radiation is one of the most important topics of climate research. In recent years it has been recognized that only full three-dimensional (3D) treatment of this interaction can provide answers to many climate and remote sensing problems, leading to worldwide independent development of numerous 3D radiative transfer (RT) codes. The "International Intercomparison of 3-Dimensional Radiation Codes", or I3RC, described in this paper, sprung from the natural need to compare the performance of these 3D RT codes used in a variety of current scientific work in the atmospheric sciences. I3RC supports intercomparison and development of both exact and approximate 3D methods in its effort to: (1) understand and document the errors and limits of 3D algorithms and their sources; (2) provide ÒbaselineÓ cases for future code development for 3D radiation; (3) promote sharing and production of 3D RT tools; (4) derive guidelines for 3D RT tool selection; and (5) improve atmospheric science education in 3D RT. Results from I3RC have been presented in two workshops, and are expected to guide improvements in both remote sensing and radiative energy budget computations in cloudy atmospheres.