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.