Recent investigation, mainly by nuclear magnetic (n.m.r.) and electron spin (e.s.r.) resonance spectroscopy, provide experimental support for the view that both antenna or light-harvesting chlorophyll and photo-reaction centre chlorophyll are aggregated species, but are aggregated in quite different ways. The spectral properties of antenna chlorophyll appear to be best explained in terms of chlorophyll oligomers, (Chl 2 ) n , in which the chlorophyll molecules are bound to each other via keto C = 0 • • • Mg coordination interactions, whereas light conversion occurs in special pairs of chlorophyll molecules probably held in the necessary configuration by a water molecule between them: [Chl H 2 O Chl]. Photo-reactive bacteriochlorophyll a appears to have a very similar special pair structure, but antenna bacteriochlorophyll a appears to be a much more complex entity than is the antenna in green plants, and it is possible that a variety of bacteriochlorophyll a -water adducts are involved in antenna behaviour. Considerations of photosynthetic membrane structure suggest that antenna chlorophyll and the special pair photo-active chlorophyll may occupy the annular region of the thylakoid lipid bilayer.