Plastics, a general term for synthetic polymeric materials, have many qualities that make them ideal for use in museums. They are stronger, lighter, and more ductile than many other materials and they can easily be made into a variety of forms from the thinnest bags to the sturdiest construction materials. However, plastics can be a source of contamination. The plastic polymer can degrade and release small molecules. The residual starting materials from polymerization and production can be released, or additives and/or by-products of additives can be released into the collection environment. Plastics made from the same type of polymer can vary greatly in quality and suitability, depending on the production method. No single plastic is perfect for all applications. Choosing a plastic for use with collection objects requires careful consideration. This chapter provides guidance in making the best choices, contains a summary of information about commonly used plastic materials, and discusses the ways in which unwanted components may be present even in plastics considered to be the most stable.