The enzyme peroxidase is a heme or iron-porphyrin protein that belongs to a large family of enzymes called the oxidoreductases. Their function mainly is to oxidize molecules at the expense of hydrogen peroxide. They are widely distributed in living organisms, and usually show dramatic colour-product formation as a result of their catalytic effect. They generally catalyse many oxygen transfer reactions involving hydrogen peroxide or anyone of the many other peroxides as electron acceptors and substrates. This ability of reducing peroxides at the expense of electron donating substrates is what marks peroxidases as ubiquitous and very important enzymes with many biotechnological applications. Not surprisingly therefore peroxidases play many important roles in different areas of biotechnology. Among others, these include such diverse areas as bioenergy, bioremediation, dye decolorization, humic acid degradation, paper and pulp and textile industries among many others. An important reason for this ability is the different areas from which peroxidases could be sourced as the function of many peroxidases show variations according to its source. This is a character that differentiates peroxidases from many other biological catalysts. Among the many different types of peroxidases are the heme peroxidases which mainly come from plants and fungi and include among others lignin peroxidases, manganese peroxidases and versatile peroxidases. Some important types of peroxidases from humans and animals are glutathione peroxidase, thyroid peroxidase, lactoperoxidase, salivary peroxidase and thyroid peroxidase.