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 about inadvertent exposure and the potential for adverse effects on human health and the environment. This chapter provides an overview of metal and metal oxide nanoparticles, their 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 assessment of the material properties is required for a full understanding of nanomaterial toxicity; a section of the chapter is therefore devoted to physicochemical characterization. This is followed by a detailed description of the current knowledge concerning 12 of the most important metal and metal oxide nanoparticles, with a systematic evaluation of invitro (cell culture) and invivo (animal) toxicity studies. Ecotoxicological effects are not discussed because the chapter is focused on implications for human health.