The occurrence of anthropogenic substances in the biotic environment is considered a major threat to biodiversity and ecosystem functioning. Plastic fragments are a typical example of anthropogenic elements that have a multitude of direct and indirect impact on organisms and environmental processes. Due to its poor biodegradability, plastic accumulates in the environment and decomposes into smaller particles (“nanoplastics”). Within the framework of this chapter, nanoplastics or plastic particles smaller than 1 μm in size will be discussed. Nanoplastics can be found in a variety of different environments (e.g., in the water, air, soil) and can even be transferred in food chains. In comparison to larger plastics, nanoplastics have a greater bioavailability and a larger surface and contact area compared to their weight. Hence, nanoplastics are known to be taken up in organisms and even into cells by a wide variety of organisms (e.g., plants or mammalian cells). Tissue translocations have also been shown. However, the risk estimation is still hindered by a knowledge gap of the attributes of environmental nanoplastics such as concentration, composition, morphology, and size, caused by technical limitations in detection and quantification. Uptake and effects of nanoplastic-particles were listed for several levels of biological organization and common used test organisms, subdivided into unspecific effects (nanoparticle effects) and specific effects (nanoplastic effects). In particular, several internal effects due to the uptake of nanoplastics were shown, from molecular over physiological to whole organism responses.