Many engineered nanomaterials (ENMs) have extraordinary novel properties and do not have naturally occurring analogs. It is therefore prudent to assess their potentially harmful impacts in parallel with their rapid development. Forecasting the potential risks from ENMs requires an ability to predict ENM fate and bioavailability in the environment. ENMs are highly dynamic in environmental and biological systems, making it important to understand the range of possible geochemical transformations of ENMs and how those transformations affect their properties, bioavailability, and toxicity.Important transformations include adsorption of biomacromolecules, biodegradation, oxidation/reduction reactions, and sulfidation. Laboratory and field scale studies describing these transformations, and their effects on ENM fate, bioavailability, and toxicity are discussed.