Metal Oxo and Metal Peroxo Species in Catalytic Oxidations The present volume is a description of the current knowledge on the different metal oxo and metal peroxo species which are involved in catalytic oxidations. These highly reactivity metal oxygen species are the key intermediates in heme or non heme oxygenases as well as in metal catalyzed oxidations. The accumulated knowledge on these reactive entities will facilitate a better understanding of the different intermediates involved in the catalytic cycles of enzymes like cytochromes P 450 non heme oxygenases copper oxygenases in the oxidation of water by the manganese center of Photosystem II in the chemistry of dioxiranes and also in DNA cleavage performed by activated bleomycin. The description of these metal oxo and metal peroxo species is based on physico chemical data reactivity results and also on theoretical calculations