The increased application of nanoparticles (NPs) is increasing the risk of NPs being released into the environment. Although many toxicity studies have been conducted, the environmental risk is difficult to estimate, because uptake mechanisms are often not determined in toxicity studies. Here we review the dominant uptake mechanisms of NPs in cells, as well as the effect of NP properties, experimental conditions and cell type on NP uptake. Knowledge about NP uptake is crucial for risk assessment, and essential to predict the behavior of NP, based on their physical-chemical properties. Important uptake mechanisms for eukaryotic cells are macropinocytosis, receptor-mediated endocytosis and phagocytosis in specialized mammalian cells. The studies reviewed demonstrate that uptake into non-phagocytic cells depends strongly on NP size, with an uptake optimum at an NP diameter of about 50 nm. Increasing surface charges, either positive or negative, have been shown to increase particle uptake in comparison to uncharged NPs. Another important factor is the degree of (homo-) aggregation. Results regarding shape have been ambiguous. Difficulties in the production of NPs, with one property changed at a time, call for the full characterization of NP properties. Only then it is possible to draw conclusions which property affected the uptake. Environ Toxicol Chem © 2013 SETAC.