The "law of purity", or "Reinheitsgebot" has for the last 492 years and up till today exactly regulated what a german beer had to made of and ensured its world reputation.
Duke Wilhelm the VI of Bavaria issued the law in 1516 :
"In all cities, markets and the country the only ingredients used for the brewing of beer must be barley, hops and water.......
This makes it the oldest food quality regulation in history and the reason why it became nationwide law is that Bavaria made the maintenance of beer quality a precondition for joining the german national state in 1871. Seriously

492 Years of Good Beer: Germans Toast the Anniversary of Their Beer Purity Law - International - SPIEGEL ONLINE - News