Depends on what type of anarchy. If you're talking about something like [url=http://en.wikipedia.org/wiki/Anarcho-capitalism]anarcho-capitalism[/url], then everything is enforced (in this case, contracts between sovereign individuals) by privately-operated institutions (subscription-based police forces, private courts, etc.) Of course, such a system has the same fundamental problem as true communism: it only works if [i]everyone[/i] willingly follows it.