This book explores in detail ribonucleotide reductase (RNR), an essential enzyme in cell biology and pharmacology. It begins with an introduction to RNR, describing its three-dimensional structure, properties, and the metals required for its function. The book explores the complex enzymatic mechanism of RNR, including its catalytic processes, regulation and reaction intermediates. The physiological role of RNR is analyzed, including its importance in DNA replication, associated pathologies such as cancer, and studies using animal models. Pharmacological applications are examined, with a discussion of inhibitors used in cancer therapy and the challenges of toxicity. The book also discusses techniques for structure determination, such as X-ray crystallography, NMR and cryo-EM, comparing their advantages and limitations. Finally, it looks to the future with perspectives on technological developments, potential impacts, and emerging innovations in the study of RNR.