Laccase is a blue copper-containing oxidase which is widely distributed in higher plants and fungi. This chapter discusses the knowledge of the molecular properties of laccase, particularly the relationship between structure and function of these enzymes. It suggests that the transient species might be an important intermediate in the catalytic reaction of this enzyme. Laccase belongs to a small group of blue oxidases which can utilize the full oxidizing capacity of dioxygen and reduce it to two molecules of water. Laccase-like enzymes has been prepared from diverse plant tissues as peaches and tea leaves. Fungal laccase has been proposed to be involved in the formation of humic acids in the soil. The main sources for laccase are, however, many different fungi, e.g., the Basidiomycetes Polyporus, Pleurotus, and Pholiota and the Ascomycetes Neurospora, Podospora, and Aspergillus. Several chromatographic forms of laccase are obtained from both tree and fungal sources.