Engineered nanomaterials hold great potential in many sectors of society, not least in medicine. However, the increasing production and use of engineered nanomaterials also raises concerns regarding inadvertent exposure and the potential for adverse effects on human health. The chapter provides an overview of metal and metal oxide nanoparticles, their principal applications, and the potential for human exposure. This is followed by a discussion of general principles of nanoparticle-induced toxicity and methods for toxicity testing of nanomaterials. Careful characterization of material properties is required for a full understanding of nanomaterial toxicity and a section of the chapter is devoted to physicochemical characterization. This is followed by a comprehensive description of the most common metal and metal oxide nanoparticles, with a systematic evaluation of in vitro and in vivo toxicity studies. Finally, an overview of emerging, two-dimensional (2D) metal nanomaterials is provided.