Excitation-emission maps were constructed by measuring emission spectra from tobacco thylakoids and from thylakoids and intact cells of the cyanobacterium Synechocystis 6803. The measurement of such maps is greatly facilitated by the current diode-array detector technology. We show that excitation-emission maps are valuable tools for studies of the structure and energy transfer pathways in photosynthetic systems. and Mika Keränen, Eva-Mari Aro, Esa Tyystjärvi.