This chapter provides an in-depth review of copper (Cu) enzymes, focusing on their enzymology, kinetics, geometric and electronic structures, and reaction mechanisms. The review highlights the role of Cu in electron transfer (ET), O2 binding and activation, NO2− and N2O reduction, and substrate activation. It discusses the importance of Cu(I) and Cu(II) sites in these enzymes, including their spectroscopic and electronic properties. The chapter covers various spectroscopic techniques such as X-ray absorption spectroscopy (XAS), K-β emission, electron paramagnetic resonance (EPR), and X-ray magnetic circular dichroism (XMCD). These techniques are used to study the coordination environment, ligand field theory, and excited states of Cu sites. The text also explores the magnetic coupling between Cu(II) centers in binuclear Cu sites, including zero-field splitting (ZFS) and antiferromagnetic coupling. The chapter emphasizes the significance of these properties in understanding the catalytic mechanisms of Cu enzymes, particularly in O2 activation and reduction processes.This chapter provides an in-depth review of copper (Cu) enzymes, focusing on their enzymology, kinetics, geometric and electronic structures, and reaction mechanisms. The review highlights the role of Cu in electron transfer (ET), O2 binding and activation, NO2− and N2O reduction, and substrate activation. It discusses the importance of Cu(I) and Cu(II) sites in these enzymes, including their spectroscopic and electronic properties. The chapter covers various spectroscopic techniques such as X-ray absorption spectroscopy (XAS), K-β emission, electron paramagnetic resonance (EPR), and X-ray magnetic circular dichroism (XMCD). These techniques are used to study the coordination environment, ligand field theory, and excited states of Cu sites. The text also explores the magnetic coupling between Cu(II) centers in binuclear Cu sites, including zero-field splitting (ZFS) and antiferromagnetic coupling. The chapter emphasizes the significance of these properties in understanding the catalytic mechanisms of Cu enzymes, particularly in O2 activation and reduction processes.