Nanotechnology and the production of new materials with different applications in the biomedical, pharmaceutical or environmental fields are growing exponentially in the last decade. However, the toxicological impact and possible risks for the environment and human health are nowhere near known and understood. Different parameters such as the method of synthesis, size, surface that play an important role in the applicability of new materials could have adverse effects on people, animals and the environment. As a result, a correct characterization of nanomaterials and the understanding of the processes that take place on their surface when they are in contact with living systems is vital for the prevention of possible toxicological effects. This special issue aims to provide as much information as possible regarding the toxicology of nanoparticles and their impact on living organisms and the environment. Moreover, the dose is a very important key parameter both in identifying and evaluating the possible risks of nanoparticles. Information about the ways of entry of nanoparticles into the body as well as the interaction of nanoparticles with living organisms could have a major contribution in the implementation of regulations regarding the possible toxicological potential of some nanomaterials.