This chapter will focus on major points concerning the safety of magnetic nanoparticles (MNPs) from a toxicological point of view and will provide key references to help the reader to delve further into the subject. It is important to know that toxicology as a discipline has been relatively new, since Archiv fur Toxicologie, the šrst journal expressly dedicated to experimental toxicology, started publication in Europe in 19301. The Society of Toxicology, the pre-eminent toxicology organisation, was not founded until 19612. The reader is referred to more reviews on the history of toxicology3 and regulatory toxicology4. Assessing the potential hazards of nanomaterials is an emerging area in toxicology and health risk assessment. Nanotoxicology is a subspecialty of particle toxicology. It addresses the toxicology of nanoparticles (<100 nm diameter), which appear to have toxic effects that are unusual and not seen with larger particles. Because of the quantum size effects and large surface area to volume ratio, nanomaterials have unique properties compared with their larger counterparts. Nanotoxicological studies are intended to determine whether and to what extent these properties may pose a threat to the environment and to human beings5.