Although the theory of partial coherence was formulated in a reasonably general form about a quarter of a century ago, it was not until a few years ago that this theory began to be applied to problems of radiation from partially coherent sources. In this review article, the properties of the radiant intensity generated by a planar source of any state of coherence will be discussed. It will be first recalled that the radiant intensity can be expressed as a two-dimensional spatial Fourier transform of a correlation function of the field in the source plane, averaged over the source area. The characteristics of the radiation from several model sources will then be analyzed. With the help of these results, certain equivalence theorems relating to the radiant intensity from planar sources of entirely different degrees of spatial coherence will be reviewed and the underlying physical principles will be elucidated. A number of illustrative examples will also be given. Finally some very recent work, which has led to the construction of planar sources with controllable degrees of spatial coherence, will be described. Experiments carried out with these sour-ces will be discussed; they verify the main relationships between the coherence properties of the source and the directionality of the light it generates.