The chapter about Wood Decay Fungi dives deeply into the intriguing realm of these microorganisms, which assume a crucial function in the natural process of recycling lignocellulosic material. Wood decay fungi, which cover a wide range of taxonomic groupings, play a significant role in the decomposition process by effectively breaking down the intricate composition of wood and reintroducing its components back into the ecosystem. The present chapter explores the ecological importance of wood decay fungi, providing a detailed analysis of their complex mechanisms involved in the enzymatic breakdown of lignin, cellulose, and hemicellulose. The chapter additionally emphasizes the economic and environmental consequences associated with wood decay fungi. This includes their various functions within forest ecosystems, their impact on the timber industry, and their prospective applications in biotechnology and bioenergy. Furthermore, this study investigates the difficulties presented by wood decay fungus, specifically within the field of wood preservation and biodeterioration. This chapter lays out a detailed summary of the critical ecological benefits, biological diversity, and various ways wood decay fungi influence the surrounding environment as we solve the mysteries associated with them. The comprehension of wood decay fungus is a crucial aspect in our pursuit to gain knowledge about the complexities of the natural world, whether for scientific investigation, industrial advancement, or conservation endeavours.