The subject of this book is the amazing enzyme ribonucleotide reductase (RNR), the enzyme responsible for the conversion of ribonucleotides to deoxyribonucleotides. The prerequisite for DNA-synthesis and DNA-repair in all living cells is the supply of the four deoxyribonucleotides. Such molecules result from the enzymatically difficult radical-induced reduction of ribonucleotides, a multistep chemical process catalyzed by RNR. RNR was the first enzyme in which the presence of an amino acid radical (a tyrosyl) in E. coli Class Ia RNR has been proven; since then several other biological amino acid radical species have been found on e.g. tryptophan, glycine, cysteine, lysine residues and on amino acid derived small cofactors like 2 tryptophanes in thryptophan-trypthanyl-radical or cysteine-tyrosyl-radical in other enzymes. As all known cellular life forms store their genetic information as DNA, RNR is likely to be found in all growing cells of every living organism, a fact that is confirmed by a rapidly increasing number of genomic screenings.