Usage of nanoparticles holds geat potential in many technical as well as medical applications. Metal and metal oxide nanoparticles are often used as catalysts in industries for improving the functional properties of the product. Various fields, like medical imaging and biology, environmental remediation, sensors and biomedicine, are benefitting from the recent advanced nanotechnology. Conversely, these developments are connected with unwanted risks and uncertainties. The same properties which can be beneficial from a technical point of view can be harmful in another aspect. Controversies still remain with respect to the toxic effects and mechanisms of these nanoparticles. In order to avoid such unnecessary risks and to facilitate the use of nanoparticles effectively, there is a need for ample toxicological research as well as risk assessment of the nanoparticles. The relation between the properties of material and its toxicity seems to be complex and difficult to understand, which can be partly due to the incomplete characterization of the nanomaterial and experimental artifacts in the nanomaterial’s characterization and its interaction with living organisms. Nanotoxicology, which studies nanoparticle interaction with the biological system and measures the level of toxicity related to human health, is the focus of many researchers. In this chapter, we focus on the hazards or toxicity of the nanoparticles and more specifically the metal- and metal oxide-containing nanoparticles. The mechanism underlying the toxicity of the nanomaterials is discussed herein. This includes the generation of relative oxygen species resulting in the subsequent formation of oxidative stress, leading to DNA damage, apoptosis and initiation of cancer. In addition, critical factors, like size, shape, particle dissolution, UV light activation, aggregation, etc., and future prospects of nanoparticles are also discussed. © 2018 Scrivener Publishing LLC.