 Nonomaterials, NMs, have gained prominence in technological advancements due to their tunable physical, chemical, and biological properties with enhanced performance over their bulk counterparts. They can be categorized according to their size, composition, shape, and origin. The ability to predict the unique properties of NMs increases the value of each classification. As the production of NMs and their industrial applications increase, so do the potential environmental risks posed by them. This review compares synthetic, engineered, and naturally occurring nanoparticles, NPs, and nanostructured materials, NSMs, identifying their nanoscale properties and defining the specific knowledge gaps related to the risk assessment of NPs and NSMs in the environment. The review provides an overview of the history and classifications of NMs, discusses the various sources of NPs and NSMs, from natural to synthetic, and their toxic effects on mammalian cells and tissues. Additionally, it examines the types of toxic reactions associated with NPs and NSMs, and the regulations implemented by different countries to reduce the associated risks. This article was authored by Jason Givanandam, Ahmed Baram, Yen S. Chan, and others.