Decomposer fungi represent an important biological resource that can be utilized in various environmental and agricultural applications, in addition to their natural role in breaking down organic waste and recycling nutrients. Their ability to produce a wide range of enzymes has generated interest in using selected fungal species for waste treatment, improving soil properties, and supporting certain bioremediation technologies. The environment contains numerous fungal species involved in organic matter decomposition, including species of Aspergillus, Penicillium, and Trichoderma, as well as fungi specialized in wood degradation. These species differ in their enzymatic capabilities and the materials they can decompose. Their applications also vary depending on the characteristics of each species or strain and the conditions under which they are used. White-rot fungi, including species of the genus Phanerochaete, play an important role in degrading lignin found in woody tissues. These fungi can produce oxidative enzymes that help break down certain complex organic compounds, contributing to the decomposition of wood and plant residues and the recycling of carbon and associated elements. These capabilities provide a scientific basis for exploring the use of selected fungi in treating plant waste. In organic fertilizer production, fungi contribute to the decomposition of plant residues and agricultural waste in cooperation with bacteria and other organisms. This process helps transform waste into organic materials that can be added to soil to improve its properties, support biological activity, and contribute to the return of nutrients to agricultural systems. Selected fungi can also be used to process crop residues, straw, and wood, helping reduce waste accumulation and convert some of these materials into resources suitable for agricultural or industrial applications. The efficiency of these applications depends on the fungus's ability to degrade the components of the waste and on the environmental conditions under which treatment takes place. Another important application is bioremediation (Bioremediation), in which certain fungi produce oxidative enzymes that contribute to the breakdown of specific organic pollutants, including some dyes and complex aromatic compounds. However, the success of this approach depends on the nature and concentration of the pollutant, the fungal species used, and environmental conditions. This does not mean that all pollutants can be removed through fungal activity. In agriculture, decomposer fungi can help maintain soil biological activity by breaking down organic residues and recycling nutrients. Mycorrhizal fungi perform a somewhat different function, forming associations with plant roots and helping plants absorb certain soil nutrients, particularly phosphorus. Selected strains of Trichoderma are also used in some agricultural applications and biological control programs, depending on their characteristics and intended purpose. These applications align with the concept of the circular economy (Circular Economy), which aims to reduce waste, utilize available resources efficiently, and transform waste materials into valuable products. Using fungi to convert certain organic wastes into compost or intermediate materials for industrial applications can improve resource efficiency and reduce waste. Despite the promising potential of fungi in these fields, their practical applications require the selection of appropriate species and strains and the evaluation of their effectiveness and safety under actual operating conditions. Successful implementation also requires consideration of the properties of the material being treated, environmental conditions, and the specific application to ensure the desired benefits are achieved. The use of decomposer fungi in environmental and agricultural applications represents an important pathway for harnessing the biological capabilities of microorganisms. It combines an understanding of natural processes with the development of practical approaches to waste management, soil improvement, and environmental sustainability. Al-Mustaqbal University – The First University in Iraq