An attribute of values which are used exactly once: they are neither duplicated nor destroyed. Such values require no garbage collection, and can safely be updated in place, even if they form part of a data structure. Linear types are related to the linear logic of J.-Y Girard. They extend Schmidt's notion of single threading, provide an alternative to Hudak and Bloss' update analysis, and offer a practical complement to Lafont and Holmström's elegant linear languages.