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Publisher Summary This chapter focuses on the use of additives to design and improve the performance of multilayer flexible packages. The two principal approaches of modifying the properties of a polymeric substrate are structural modification of the polymer and incorporation of additives. Incorporation of additives has two general possibilities for multilayer films. First, the additives can be incorporated into the organic substrate after synthesis and second, the additives can be incorporated into the layers of a multilayer film. Additives can be incorporated at various points during the life time of a polymer. The specific additive and its most appropriate physical form have an impact on the most appropriate addition point during a substrate's life cycle. It is important to match the additive's physical form that will be used in handling and dosing to the industrial plant requirements. While manufacturing multilayer films, it should be highlighted that not every additive needs to be added to every layer of the construct. Some additives, such as UV additives, need only to be added to the skin layer. In the case when additives are added to selected layers only, migration effects must be considered as migration into neighboring layers will decrease the additive concentration in the initial layer. The term “antioxidant” is used to describe all stabilizer classes that are utilized to protect organic substrates from thermal and thermooxidative degradation during storage, processing or conversion, and service life in the absence of ultraviolet light and maintain its initial properties.