Titanium dioxide (TiO 2) nanoparticles (NPs) are manufactured worldwide in large quantities for use in a wide range of applications. TiO 2 NPs possess different physicochemical properties compared to their fine particle (FP) analogs, which might alter their bioactivity. Most of the literature cited here has focused on the respiratory system, showing the importance of inhalation as the primary route for TiO 2 NP exposure in the workplace. TiO 2 NPs may translocate to systemic organs from the lung and gastrointestinal tract (GIT) although the rate of translocation appears low. There have also been studies focusing on other potential routes of human exposure. Oral exposure mainly occurs through food products containing TiO 2 NP-additives. Most dermal exposure studies, whether in vivo or in vitro, report that TiO 2 NPs do not penetrate the stratum corneum (SC). In the field of nanomedicine, intravenous injection can deliver TiO 2 nanoparticulate carriers directly into the human body. Upon intravenous exposure, TiO 2 NPs can induce pathological lesions of the liver, spleen, kidneys, and brain. We have also shown here that most of these effects may be due to the use of very high doses of TiO 2 NPs. There is also an enormous lack of epidemiological data regarding TiO 2 NPs in spite of its increased production and use. However, long-term inhalation studies in rats have reported lung tumors. This review summarizes the current knowledge on the toxicology of TiO 2 NPs and points out areas where further information is needed.