Message from the Guest EditorThe safety of nanomaterials, despite the benefits they bring to the society, is still a matter of debate. Nanomaterials (NMs) can easily enter the human body and cross all intrinsic barriers, making them very useful as drug delivery vectors; on the other hand, nanomaterials may interact with biological systems and induce potentially harmful effects. Presently, there is increasing concern about the detrimental health effects due to NM exposure. NMs have been reported to induce oxidative stress, DNA damage, inflammation, and many other adverse effects which are known to be crucial for the development of lifestyle diseases. Mitochondria malfunction, leading to the generation of free radicals and subsequent oxidative stress, seems to be a major cause of NM toxicity. The disturbance of cellular redox equilibrium may have differing effects depending on the magnitude of oxidative stress—from triggering signal transduction via low concentrations of ROS through to the induction of oxidative damage to cellular components and organelles as well as the induction of cell death resulting from immense ROS production. This, in turn, usually affects tissue and organ function.