Over the past two decades, the scientific and business sectors have shown a comparable level of interest in nanocomposites and nanofillers as there has been in nanotechnology in general. Nanoparticles (NPs) and nanofillers (NFs) are increasingly being used in a wide range of technical advances, from biomedical devices to automotive product portfolios. NFs are a preferred filler over microfillers due to their potential physical properties, such as large surface areas, high surface energy, and less structural defects. Different characteristics, including wettability and rheology, distinguish NPs from NFs. Additionally, there are functional advantages that are necessary for a variety of applications, like electrical and thermal conductivity. When used in conjunction with conventional fillers, nanofillers have the potential to greatly enhance or modify the many properties of the materials they are mixed into, including the optical, electrical, mechanical, thermal, and/or fire-retardant capabilities. The characteristics of NPs and NFs are covered in this chapter, which from a toxicological standpoint may have an impact on the environment and human health.