Unprecedented levels of pollutants in air and water resources resulting from improper waste disposal have raised considerable concerns about their potential impacts on human health and the environment. Therefore, the development of technologies to be employed in air and water filtration and separation systems that can help mitigating pollutant-related issues is highly demanded. In this regard, scientific and technological efforts have been focused on developing novel functional nanomaterials that combine high surface area, tailored functionalities, permeability, and low-pressure drop to be used in water remediation and air filtration processes. Nanofibrous membranes produced by electrospinning stand out for such applications, owing to their remarkable features as high specific surface area, high porosity, high permeability, and mechanical flexibility. In addition, their surface can be functionalized to improve their filtration efficiency. In this chapter, a review on different types of composite electrospun nanofibers modified with nanoparticles (metal oxide, metallic, and carbonaceous nanoparticles) and 2D materials is presented. Such fibers are designed to yield low-cost, scalable, and efficient air and liquid filtration and separation systems for industrial, medical, and environmental applications.