Green synthesis of metal and metal oxide nanoparticles has gained significant attention in materials science as a sustainable and eco-friendly method for producing a wide range of materials, including metal and metal oxide nanoparticles, hybrid materials, and bioinspired materials. This review summarizes the fundamental processes and mechanisms of green synthesis, particularly for metal and metal oxide nanoparticles using natural extracts. The role of biological components, essential phytochemicals such as flavonoids, alkaloids, terpenoids, amides, and aldehydes as reducing agents and solvent systems is explored. The stability and toxicity of nanoparticles, along with surface engineering techniques for achieving biocompatibility, are also discussed. Applications of these nanoparticles in environmental remediation, including antimicrobial activity, catalytic activity, pollutant dye removal, and heavy metal ion sensing, are covered. Green synthesis methods, such as bacterial, fungal, yeast, and plant-based approaches, are highlighted for their efficiency in producing nanoparticles. These methods utilize natural resources and biological components to reduce metal ions into nanoparticles, often with lower energy consumption and environmental impact. The use of plant extracts is particularly effective for large-scale nanoparticle production compared to bacterial or fungal methods. The stability and toxicity of nanoparticles are influenced by factors such as particle size, surface capping, and functionalization. Green synthesis also offers advantages in terms of cost-effectiveness, environmental friendliness, and the ability to produce nanoparticles with controlled shapes and sizes. In environmental remediation, metal and metal oxide nanoparticles exhibit antimicrobial activity by disrupting microbial cell membranes and metabolic processes. They are also effective in catalytic applications, such as the reduction of 4-nitrophenol to 4-aminophenol. Additionally, these nanoparticles are used for the removal of pollutant dyes and heavy metal ions from water, demonstrating their potential in addressing environmental pollution. The use of metal oxide semiconductor nanoparticles, such as ZnO and TiO₂, is particularly promising for photocatalytic degradation of dyes and pollutants. Green synthesis methods are also being explored for their potential in heavy metal ion sensing, with metal nanoparticles showing selective and sensitive detection capabilities. Overall, green synthesis of metal and metal oxide nanoparticles offers a sustainable and eco-friendly alternative to traditional methods, with significant applications in environmental remediation and other fields. Future research should focus on scaling up green synthesis methods and addressing challenges related to nanoparticle stability, toxicity, and environmental impact.Green synthesis of metal and metal oxide nanoparticles has gained significant attention in materials science as a sustainable and eco-friendly method for producing a wide range of materials, including metal and metal oxide nanoparticles, hybrid materials, and bioinspired materials. This review summarizes the fundamental processes and mechanisms of green synthesis, particularly for metal and metal oxide nanoparticles using natural extracts. The role of biological components, essential phytochemicals such as flavonoids, alkaloids, terpenoids, amides, and aldehydes as reducing agents and solvent systems is explored. The stability and toxicity of nanoparticles, along with surface engineering techniques for achieving biocompatibility, are also discussed. Applications of these nanoparticles in environmental remediation, including antimicrobial activity, catalytic activity, pollutant dye removal, and heavy metal ion sensing, are covered. Green synthesis methods, such as bacterial, fungal, yeast, and plant-based approaches, are highlighted for their efficiency in producing nanoparticles. These methods utilize natural resources and biological components to reduce metal ions into nanoparticles, often with lower energy consumption and environmental impact. The use of plant extracts is particularly effective for large-scale nanoparticle production compared to bacterial or fungal methods. The stability and toxicity of nanoparticles are influenced by factors such as particle size, surface capping, and functionalization. Green synthesis also offers advantages in terms of cost-effectiveness, environmental friendliness, and the ability to produce nanoparticles with controlled shapes and sizes. In environmental remediation, metal and metal oxide nanoparticles exhibit antimicrobial activity by disrupting microbial cell membranes and metabolic processes. They are also effective in catalytic applications, such as the reduction of 4-nitrophenol to 4-aminophenol. Additionally, these nanoparticles are used for the removal of pollutant dyes and heavy metal ions from water, demonstrating their potential in addressing environmental pollution. The use of metal oxide semiconductor nanoparticles, such as ZnO and TiO₂, is particularly promising for photocatalytic degradation of dyes and pollutants. Green synthesis methods are also being explored for their potential in heavy metal ion sensing, with metal nanoparticles showing selective and sensitive detection capabilities. Overall, green synthesis of metal and metal oxide nanoparticles offers a sustainable and eco-friendly alternative to traditional methods, with significant applications in environmental remediation and other fields. Future research should focus on scaling up green synthesis methods and addressing challenges related to nanoparticle stability, toxicity, and environmental impact.