Engineered nanomaterials are widely used in consumer products such as cosmetics, paints, fabrics, and electronics. Because of their small size (diameter <100 nm), they often have unusual properties. Once released into the human body or the environment, they are also fiendishly difficult to find again. In 2006, Nel et al. described possible mechanisms by which engineered nanomaterials interact with biological entities and the toxicological responses that may be triggered (1). Despite much research since then, mechanistic understanding remains limited. Evidence for acute toxicity from nanomaterials at realistic doses is limited; there also is no simple correlation between toxic responses and nanoparticle size or other predictable pattern of toxicity. For answers to emerge, the nanosafety community must embrace recent technical advances and build consensus on testing methodologies.