The process of electronic energy transfer between chromophoric molecules is often used to estimate distances within and between macromolecules, as well as molecular aggregates. This review is focused on the case of transfer within pairs of identical chromophores, which is hereafter referred to as donor-donor energy migration (DDEM). The majority of bichromophoric donor-donor molecules hitherto synthesized is presented. To monitor DDEM fluorescence, depolarization experiments, such as the measurement of fluorescence anisotropy, are needed. Since the DDEM process, and the reorientation of the donor molecules both contribute to the fluorescence anisotropy the interpretation of data becomes very complex. A recently presented model of DDEM for this case is described together with experimental support for its validity. The DDEM method may provide a powerful tool for examining questions related to structure and function of protein systems, for which one cannot use the traditional methods of X-ray diffraction and multi-dimensional NMR. Technical aspects of DDEM and its potential applicability are briefly discussed.