The FDA, which is responsible for ensuring the safety of most of the U.S. food supply, is not required to review substances, such as spices and preservatives, added to food that are generally recognised as safe (GRAS) for their intended use. Currently, companies may determine a substance is GRAS without FDA's approval or knowledge. However, a few substances previously considered GRAS have later been banned and concerns have been raised about the safety of other GRAS substances, including those containing engineered nanomaterials. This book provides an overview of the FDA's regulation of GRAS substances, which are exempt from the premarket approval process for food additives, and discusses the FDA's proposed rule.