This chapter focuses on the role of both natural and synthetic substances that are added to foods and provides an integrating view of their functionalities. It discusses the use of acids and acid salts in chemical leavening systems, the role of specific acidic microbial inhibitors in food preservation, and the function of acids as chelating agents. The chapter also discusses one of the most important functions of acids in foods is participation in buffering systems. Chemical preservatives with antimicrobial properties play an important role in preventing spoilage and assuring safety of many foods. Antioxidants often exhibit variable degrees of efficiency in protecting food systems, and combinations often provide greater overall protection than can be accounted for through the simple additive effects. Thiodipropionic acid and dilauryl thiodipropionate remain as approved food antioxidants, even though removal of the compounds from the approved list has been proposed because they were not being used in foods.