In the modern era, research on the synthesis of nanoparticles (NPs) has been growing exponentially. Due to their small size together with extra-ordinary physico-chemical properties, a variety of NPs i.e., metallic, carbon-based, fluorescent, and polymer-based have been exploited in different fields such as tissue engineering, drug delivery, and various other therapeutic applications. Instead of multi-disciplinary applications of NPs, research dealing with the toxicity concerns and influence of such materials, on the public health, plants and environment is still in its infancy. NPs can cause damage at the cellular, sub-cellular, molecular and protein levels owing to their extremely small size, large surface area to volume ratio, shape, and surface functionality. The present review is aimed to provide wide-ranging information related to NPs toxicology, the mechanisms of action, routes of their entry into the body and probable impacts on human health. Understanding of NPs entry routes into the body entails further research so as to update policymakers and regulatory bodies about the toxicity concerns associated with these nanomaterials. Proper characterization of NPs, factors affecting uptake and toxicity of NPs, as well as an understanding of processes when NPs come in contact with living beings, is critical to estimate the possible hazards.