Haem-containing peroxidases (EC 1.11.1.7) are ubiquitous and have been found in a broad range of plant and animal species, in which they have multiple molecular forms ranging from 30 to 150 kDa (from 251 to 726 aminoacid residues) and a broad subcellular distribution. Peroxidases have been implicated in a variety of physiological processes. For example, they participate in the biosynthesis and degradation of lignin, in the repair of wounds, in auxin metabolism, etc. Peroxidases are defined as oxidoreductases that catalyze the oxidation of a wide variety of organic and inorganic substrates by hydrogen peroxide or organic peroxides, their structural and catalytic essential elements being the ferriprotoporphirin IX prosthetic group, Ca²⁺ ions, and glycan-bound molecules. Circular dichroism, protein intrinsic fluorescence and differential scanning calorimetry studies have revealed the high thermostability of peroxidases over a broad pH range. Peroxidase electron transfer is a fundamental phenomenon not only in cellular processes but also in many reactions of biotechnological interest, such as organic synthesis, enzyme electrodes, biosensors, immunoassays, degradation of pollutants, etc.