Closures are the devices and packaging components that seal containers: caps, corks, crown corks, lids, stoppers, and tamper-evident designs. A closure does quiet, critical work — holding carbonation, blocking oxygen and microbes, surviving distribution, and proving to the buyer that nobody opened the pack first. Materials range from metal and plastic to natural cork, each matched to the product: a still water bottle and a pressure-fermented beer demand very different seals. Seal integrity testing and liner safety, including migration control, sit at the centre of closure quality assurance.