The need is especially acute for nanomaterials, as they present several challenges: toxicity that can be a function of both chemical and physical properties, novel toxic mechanisms, and complicated absorption, distribution, metabolism, and excretion properties and their sheer molecular complexity and diversity. Green chemistry began as a movement to steer the practice of chemical research proactively toward sustainability and benign design. Toxicology has faced on the one hand the Scylla of failing to foresee the toxicity of a compound before it is in widespread use and on the other hand the Charybdis of adopting the precautionary principle and assuming chemicals are hazardous until proven otherwise. The many ways in which nanomaterials interact with biological barriers represent a significant data gap for nanotoxicology. Toxicologists tend to think of molecules and think less of the inputs, outputs, and processes involved in industrial chemistry. Greener toxicology for green nanomaterials will require a large-scale commitment on the part of the community.