Many different signaling pathways are involved in deregulation of cell proliferation leading to cancer. Although genomic approaches successfully identified a great variety of molecules associated with cancerogenesis, other strategies must be applied to elucidate complex interactions between these molecules. One promising approach is fluorescence resonance energy transfer, a proximity-dependent fluorescence phenomenon. With the development of spectrally different fluorescent proteins and improved technologies for fluorescence measurements, this approach gains an enormous potential for future research. The fluorescence resonance energy transfer principle can be applied for studying all kinds of interactions or conformational changes, and it can also be used for microscopic visualization and subcellular localization of biochemical reactions, thereby promoting the progress of cancer research. Moreover, it can be exploited to develop sensitive and efficient drug screening systems and to design valuable diagnostic tools.