Plastics and rubbers form an increasingly significant part of our cultural heritage. These materials are already exhibiting degradation within collections, raising questions about proper storage and display. Degradation of polymeric materials can be accelerated by exposure to ultraviolet and visible light, stress, chemicals, heat, and pollutants. The degradation process is specific to the polymer, with each chemical structure susceptible to different agents of degradation. Identification of the polymer is required in order to provide an adequate environment for preservation. Complete identification is difficult and requires destructive testing; thus educated guesses, augmented with simple tests, are often the most practical method of identification. The date of manufacture, intended use, and appearance can provide important information. Based on the identification of the plastic, an appropriate conservation treatment can be planned. Solubilities and migration of components, along with other physical and chemical properties, both of the polymer and of the conservation material, must be considered. Any proposed treatment must be tested cautiously, due to the uncertainties contributed by the presence of copolymers, additives, and degree of degradation. Descriptions, history, typical uses, and chemical sensitivities of 16 common plastics are given, and a chronology of the introduction of commercial plastics is included.