Cover Photograph: Mitochondrial oxygen radicals resulting from competition for CoQ by complex I and other respiratory complexes, illustrate a kinetic theory of oxygen radical formation in eukaryotes as proposed by Speijer on pages 88–94 of this issue. The theory is used to explain the eukaryotic invention of peroxisomes, stating their primary function to be reduction of FADH2 amounts, generated as very long chain fatty acids are oxidized. It also sheds light on the reason for increased radical formation during the breakdown of fatty acids in general, and why neurons, which can not be easily replaced, thus have to forego this form of highly efficient energy generation. Last but not least, it could also explain why unsaturated fatty acids seem to be much healthier than their saturated counterparts, as they produce less FADH2 during breakdown. Mitochondrial oxygen radical formation could have been shaping evolution from the beginning of the eukaryotic lineage, and seems to be still doing so in complex vertebrates like ourselves. Image conceived and developed by Nick Dekker and Dave Speijer.